How to compress a PDF on Mac and iPhone without wrecking the pictures

You attach a PDF, press Send, and the mail app refuses: the file is too large. Gmail stops personal accounts at 25 MB per message, and Microsoft puts Outlook.com at 20 MB, counting the message itself as well as the attachment. Compressing the PDF is the obvious fix. The risk is that the "smaller" copy comes back with fuzzy scans and unreadable small print.

This guide explains what makes a PDF heavy, what each compression setting changes, and how to get a file small enough to send while it still reads the way it should.

What actually makes a PDF large

A PDF's size rarely comes from its text. A page of words with an embedded font takes a few kilobytes. The weight almost always comes from:

• Pictures. Photos, scans and screenshots, often at a far higher resolution than the page needs. A phone photo placed into a document as a 5 cm thumbnail may still carry its full 12-megapixel original.
• Whole embedded fonts. A document may carry an entire font file when it uses only a few dozen of its characters.
• Duplicates and leftovers. The same logo stored once per page, old versions of edited pages, page thumbnails and unused objects that nothing displays any more.
• Extras. Attachments, metadata and scripts.

This tells you where the safe savings are. Removing duplicates, unused data and unneeded font characters makes a file smaller without changing a single pixel. Only pictures can lose quality, and only if they are stored at more detail than anyone will see.

Pixels per inch, the number that decides sharpness

A picture's sharpness on paper or screen depends on how many pixels it has per inch at the size it is drawn, not on its total pixel count. A 4,000-pixel-wide photo printed 2 inches wide is 2,000 pixels per inch, far beyond what the eye or a printer can use.

Compression tools lower that density to a ceiling. As a reference point, Ghostscript, a widely used open-source PDF processor, describes its "ebook" preset as medium resolution and its "screen" preset as low resolution, and guides commonly put those at about 150 and 72 pixels per inch. Around 150 is generally comfortable for reading and office printing; lower densities suit screens but go soft on paper.

The second setting is JPEG quality, which decides how much fine detail survives re-encoding. Lower quality means a smaller file, and, pushed too far, visible blocks around edges and the text in scans.

Option 1, Preview on Mac

Apple's Preview can shrink a PDF without extra software. Apple's instructions:

1. Open the PDF in Preview.
2. Choose File > Export.
3. In the Quartz Filter pop-up menu, choose Reduce File Size.
4. Click Save, and give it a new name rather than replacing the original.

Apple notes that the compressed PDF "may be of lower quality than the original". The filter has no strength setting, and Apple does not document what it changes, so open the result and zoom into any scan or small print before you send it. The same Export dialog also offers Optimize images for screen and Save images as JPEG checkboxes.

Option 2, on iPhone

The Shortcuts app has an Optimize File Size of PDF action. Apple's own description of the action says it compresses the PDF's images, and that if they are already compressed it "might not have a measurable effect on file size" (quoted on MacScripter). That matches what users report: it helps with some files and barely changes others, and it gives you no control over how far it goes.

Option 3, Pandaedits on Mac and iPhone

Pandaedits has a Compress command in the editor, in Organize, and on the start screen ("Compress a PDF"). It writes a new, smaller copy and leaves your original as it is. Everything happens on your device; nothing is uploaded.

You choose one of three levels:

Three details make the choice easier:

• Text and drawings are never touched. Words, lines, charts made of shapes and fonts look identical at every level. Only pictures above the level's density are made smaller; a picture already below it is left alone.
• Every level stores the file more compactly. Duplicates are stored once, unused data is removed and fonts are trimmed to the letters used. That is why Best quality can still make a file smaller without changing how any page looks.
• You see the result before you save. Each level shows its actual size and how much smaller it is, for example "1.2 MB · −45%", measured by really producing that copy rather than estimating it. The picture that changed most is shown before and after, side by side, and you can zoom both together.

Under Advanced you can go further: convert pictures to grayscale, remove metadata, page thumbnails, attachments or JavaScript, or flatten annotations and form fields into the page. Each one is reported afterwards in a "What was done" list. Removing metadata is not the same as removing sensitive content from the page itself; for that, redact the PDF.

Files that need care

Signed PDFs

A digital signature is calculated over the exact bytes of the file, everything except the signature value itself, as the pyHanko signing library's documentation describes. Compressing rewrites those bytes, so the signature will no longer verify. Pandaedits warns you first and compresses a signed PDF only if you confirm. If the signature matters, send the original.

Password-protected PDFs

The compressed copy keeps the same password and permissions. If the document's owner has restricted changes, Small file and Balanced need the document's permissions password; without it, Best quality writes a plain copy.

Archival PDF/A files

These follow stricter rules so they can still be opened decades from now. A compressed copy keeps the file's PDF/A information, but it is not re-checked against the standard. Use Best quality if it must stay archival, and keep the original either way.

Scanned documents

Scans are where compression saves the most, and where over-compression shows first. Zoom into the smallest text in the preview before you choose Small file.

How to choose, quickly

1. Need it to fit in an email? Try Balanced first. The 25 MB and 20 MB limits apply to the whole message, and attachments are encoded for sending, which adds roughly a third to their size (base64 turns every 3 bytes into 4 characters, RFC 4648). Aim comfortably below the limit, not just under it.
2. Still too big? Try Small file and check the preview, or turn on grayscale if colour does not matter.
3. Must it look exactly the same? Use Best quality: it changes no picture, and many files still shrink.
4. Very large and still too big? Share a link from your cloud storage instead of attaching the file.

Whichever tool you use, open the compressed copy before you send it. It is the same habit as the last step of the PDF review checklist: review what you saved, not what is on the screen.