How to Compress a PDF on Mac Without Losing Quality

The 30-second version

The size of almost every large PDF is its pictures, so compressing a PDF means recompressing its pictures, and every method on this page does exactly that and nothing else. On a Mac, Preview's Reduce File Size filter does it with one fixed setting (JPEG quality 0.7, 2,400 pixels, 144 dpi on macOS 27: fine for screen, too low for print or scans); a custom filter in ColorSync Utility lets you choose the setting; on an iPhone running iOS 27, Preview's export sheet has two toggles for it; and Mochify does it from a plain-English prompt, with the text, fonts and layout guaranteed untouched. Pick by how much quality you need to keep and how many PDFs you have.

If the PDF is text only and still huge, no image method will help, and the Why your PDF is so big section explains what to do instead.

Compress a PDF in Preview on Mac

Preview can reduce a PDF's file size on any Mac or MacBook without extra software, and for a document you will read on a screen the built-in filter is usually enough. Here are the steps, with the menu names exactly as they appear in Apple's Preview guide:

1. Open the PDF in Preview.

2. Choose File > Export. (Not "Export as PDF", which has no filter menu.)

3. Make sure Format is set to PDF.

4. In the Quartz Filter pop-up menu, choose Reduce File Size.

5. Click Save. Give it a new name so you keep the original.

Apple's own note on this is one sentence: "When compressed, the PDF may be of lower quality than the original." The same export sheet also offers three checkboxes that nobody on the first page of search results mentions: Optimize images for screen, Save images as JPEG and Create Linearized PDF. The first two are a gentler route than the filter: they lower image resolution to what a screen can show and convert images to JPEG, without the filter's fixed quality setting. Linearized PDF only affects how fast the file starts displaying in a browser; it does not change the size much.

Check before you send. Open the exported file and zoom to 100% on a photo and on the smallest text. The text will be as crisp as before, for a reason the next section explains; the photo is what you are judging.

What "Reduce File Size" actually does to your images

A Quartz filter only ever touches the images inside a PDF. Text, fonts, vector drawings, links and page layout pass through unchanged, which is why a compressed PDF stays searchable and the type stays sharp whatever filter you pick, and why a text-only PDF does not get any smaller this way. Apple documents the components a filter can have in its ColorSync Utility guide: image effects (size, bit depth, interpolation and compression), color management, and PDF retouch for monochrome data. There is no component that rewrites text.

So "losing quality" means one thing: the pictures are downsampled to a maximum size and resolution and re-encoded as JPEG at a fixed quality. We read the filter's settings from the file itself on macOS 27.0 (October 9, 2026): JPEG compression at quality 0.7, a maximum image size of 2,400 pixels on the long edge, and images resampled to 144 dpi, with interpolation on. In plain terms, that is a screen-resolution setting. A photo on an A4 page comes out at roughly twice the pixels a laptop screen can show it at, and quality 0.7 is the range most photo apps call "high". For a document that will be read on a screen, the built-in filter is a reasonable choice, and the Mochify test later on this page lands on a similar resolution for the same reason.

The filter's reputation comes from an older version of it. In 2012 Macworld reported its settings as JPEG quality 0.0 and a maximum image size of 512 pixels, which turned photographs to mush and made small scanned text unreadable, and those are the numbers still being passed around in forum threads today. If your Mac is on an older system, check in under a minute: open ColorSync Utility, click Filters in the toolbar, and expand Reduce File Size. The filter is locked, but its settings are visible under Image Effects: compression quality, maximum image size and resolution. (In Terminal, plutil -p "/System/Library/Filters/Reduce File Size.qfilter" prints the same values.) If they read 0.0 and 512, build the custom filter in the next section before you compress anything you care about. If they read 0.7, 2,400 and 144, the blur complaints you have read are about older systems, about scans, or about a file that has been through the filter twice.

Two side effects follow from "re-encode every image at a fixed quality":

• Scans suffer most. A scanned page is one big picture, so downsampling it is downsampling the text itself. A 300 dpi scan resampled to the filter's 144 dpi keeps fewer than a quarter of its pixels, and small type is the first thing to go. That is the "hardly readable" result people describe. Scans want a higher resolution and maximum size than brochures do.

• It can come out bigger. If a PDF's images were already small, well-compressed JPEGs, re-encoding them at the filter's quality can produce a larger file. Forum reports of a 5 MB file growing to 5.5 MB are this effect, not a bug in your copy. The same happens when you "print to PDF" a file that was already compressed.

Keep the pictures sharp: a custom Quartz filter

The built-in filter is a screen setting. If the PDF will be printed, or it is a scan, or you want the photographs kept at more than 144 dpi, the fix is a copy of the filter with your own settings, made in ColorSync Utility, which Apple ships with every Mac. Preview then lists your filter in the same Quartz Filter menu.

1. Open ColorSync Utility (it is in Applications > Utilities, or search for it with Spotlight).

2. Click Filters in the toolbar.

3. Select Reduce File Size. It is locked, so click the Duplicate button to the left of the lock and work on the copy. Give the copy a name you will recognize in Preview, such as "Reduce File Size (sharp)".

4. Click the arrow next to the copy, then the arrow next to its Image Effects (image sampling and compression) component, and set:

   • Compression to JPEG, quality 0.7 to 0.8. The built-in filter already uses 0.7; raising it much further adds file size faster than it adds visible quality.
   • Resolution is the setting that does the work: the built-in filter's 144 dpi suits screen and email; use 200 for general use and 300 for print or scans. A long-standing rule of thumb from the Macworld piece puts an A4 page at 842, 1,684 and 3,508 pixels tall at 72, 144 and 300 dpi.
   • Maximum image size to match: the built-in 2,400 pixels covers screen use; raise it to about 3,500 for a 300 dpi A4 page, or leave it high and let the resolution setting decide.

5. Go back to Preview, File > Export, and choose your new filter from the Quartz Filter menu.

Two notes. If you want a one-click version, Apple's Automator app has an "Apply Quartz Filter to PDF Documents" action that can wrap your filter as a Quick Action; the ColorSync route above is enough for most people. And if the result still looks soft on a scan, raise the maximum image size before you touch the quality: on a scanned page it is the pixel count, not the JPEG quality, that decides whether the letters stay legible.

Compressing more than one PDF? The custom filter is a per-file, per-export job. For a folder of brochures or a month of scans, the Mochify workflow below takes the whole batch from one prompt.

Why your PDF is so big

A PDF is big because of its images, almost always, and the fix depends on which kind of image it is. The quick diagnosis:

• It came from InDesign, Pages, Keynote, PowerPoint or Word, and it has photos. The photos were placed at print resolution. A 4,000-pixel photo sitting in a two-inch box on the page is carrying roughly ten times the pixels anyone will ever see on screen. This is the common case and the one every method on this page fixes well. Our own 8-page InDesign brochure went from 4.93 MB to 1.47 MB, 70% smaller, with the photos downsampled, the text byte-identical and all nine fonts untouched.

• It is a scan. Every page is one picture, usually at 300 dpi or more, and there is no text underneath unless OCR has been run. These shrink well too, but they are the files where an aggressive filter destroys legibility, so use the custom filter or a tool that lets you set the resolution. Running OCR (iOS 27's Preview can do this on export, see below) does not shrink the file but makes it searchable.

• It is text only and still huge. Then the weight is fonts, usually full fonts embedded by a program that should have subset them, or an application that stored a background image once per page, or a file that has been saved many times with incremental updates stacked inside it. No image filter helps. Re-export it from the original application, or in Preview export it with the filter anyway: the pass-through often discards the accumulated junk even when it finds no images to shrink.

• It is a "print to PDF" of a presentation. A 2 MB deck can print to a 100 MB PDF because the printing path rasterizes effects and backgrounds. Export from the application instead of printing, then compress.

The numbers to aim for: for screen and email reading, images at 96 to 150 dpi relative to their printed size are indistinguishable from the original on a laptop or phone; 300 dpi is the convention for press printing. Mochify's PDF optimize measures resolution against how large each image is actually drawn on the page rather than how many pixels it stores, which is the right question: it is why a 150 dpi target is not a quality downgrade for a document nobody will print.

Compress a PDF on iPhone

On an iPhone running iOS 27, the Preview app can compress a PDF on its own; on iOS 26 it cannot do it as a PDF, and the Files app's "Compress" command only makes a zip. The steps below follow Apple's iPhone User Guide for iOS 27 and were checked on an iPhone running iOS 27.0 on October 9, 2026:

1. Open the PDF in Preview on your iPhone.

2. Tap the More button next to the filename, then tap Export.

3. Select PDF as the format. The options underneath change to a Resolution setting and five switches, all off by default: Create PDF/A, Create Linearized PDF, Embed Text, Optimize Images for Screen and Save Images as JPEG.

4. Turn on Optimize Images for Screen (Apple's note: "reduce image resolution for screen viewing. This creates a smaller file.") and Save Images as JPEG. Leave Resolution alone for a screen copy, or raise it if the PDF will be printed; it is the same lever as the Mac filter's dpi setting. Embed Text runs OCR on scanned pages so the PDF becomes searchable; it is worth turning on for scans and does not affect size much. PDF/A is an archival format and Linearized only changes how fast the file starts displaying in a browser; neither makes the file smaller.

5. Tap Save and choose a location.

On iOS 26 the same export sheet offers only a size slider, and only for image formats, so a PDF can be exported as a smaller JPEG or HEIC of each page but not as a smaller PDF. The Files app's long-press Compress command makes a .zip archive, which for a PDF is the same size or slightly larger, because a PDF's images are already compressed. If you are on iOS 26 and need a smaller PDF that stays a PDF, open mochify.app/flow in Safari and use the workflow below; it works the same on a phone as on a Mac. For photos rather than documents, see how to reduce photo file size on iPhone.

PDF too big to email: the limits and the way around them

The limits are lower than people think, and they include the message itself. Gmail's own help page puts the limit for personal accounts at 25 MB, after which it replaces the attachment with a Google Drive link. Microsoft's Outlook page states 20 MB for Outlook.com and other internet accounts, and a default of 10 MB for Exchange business accounts, "both the size of the attachment and the size of the email." Apple Mail offers Mail Drop when an attachment is over your provider's limit: up to 5 GB per attachment, available for 30 days.

So a PDF that is "too big to email" needs to get under 20 MB to be safe everywhere, and under 10 MB if it is going to a company address. That is well within reach of any method on this page for a document whose weight is photos. Three cases where compressing is the wrong move:

• The recipient needs it at full quality (a print file, a signed contract scan that must stay legible, a portfolio). Send a link instead: Mail Drop, Google Drive, OneDrive or Dropbox. The file arrives intact and the email stays small.

• It is already as small as its pictures allow. Zipping will not help; a PDF's images are compressed data and a zip of compressed data saves next to nothing. Split it into parts or send a link.

• It is dozens of pages of scans. Split by chapter, or compress with a filter that keeps the resolution high enough to read, then check a page at 100% before sending.

For the image-upload version of this problem, where a photo rather than a document is over a form's limit, see photo file too large to upload.

Mochify workflow: compress a PDF in one prompt

Mochify's PDF optimize recompresses only the images inside a PDF and returns a smaller PDF with the text, fonts, vector art, links and layout untouched, so the document stays searchable and the type stays exactly as sharp as it was. It works from a plain-English prompt, handles a batch as easily as one file, and needs no filter to build. In our own test (2026-10-09), a 7.2 MB JPEG exported to PDF from Preview (7.4 MB) came out at 498 KB on Mochify's defaults, 93% smaller, with the photo stored at 1,653 px wide: 150 dpi at the size it occupies on the A4 page. Pages' Best export placed the same photo at 598 ppi, so the default brought it to 1,004 px; ask for 300 dpi when the PDF is going to print. PDF compression is available on the paid plans (Seller, Pro and Growth) and on the Day Pass; it is not on the Free plan.

Same thing from the API. POST /v1/pdf?op=optimize on api.mochify.app with the PDF as the request body and Authorization: Bearer <key>; quality (default 75) sets the jpegli re-encode quality and maxDpi (default 150) the resolution target measured against the placed size, with 96 for screen and email and 300 to keep print quality; maxDimension (default 2000) caps the long edge of any image whatever its placed size. From the command line, mochify report.pdf --op optimize --dpi 150 does the same (or mochify report.pdf -p "compress this pdf", which goes through the same prompt parser as the web app), and the hosted MCP server at mcp.mochify.app exposes the operation to an AI agent. Full parameters are in the API docs.

Building the PDF rather than shrinking one? The converters for HEIC and HEIF to PDF, WebP to PDF and JPEG XL to PDF assemble images into a PDF in one step, free up to 3 images per file and 20 on the paid plans and the Day Pass. And if you need the pictures out of a PDF rather than smaller inside it, see extracting images from PDFs.

Privacy, stated precisely. Your PDF travels to api.mochify.app over HTTPS, is processed in memory and wiped immediately after the result is returned: no disk writes, no logs containing file data. It is not processed on your Mac, and it is not kept. The same holds whether you use the web app or the API.

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