Get a PDF Under Gmail's 25 MB Limit

Every figure here is quoted from the provider’s own documentation, checked in September 2026. Note the third column. It is the one that matters and the one nobody prints.

Two things fall out of that table immediately. Corporate limits are a moving target, because a Microsoft 365 administrator can set anything between 1 MB and 150 MB and you have no way to know which. And the default you are most likely to hit is not the headline 25 MB: it is a 20 MB message limit on an Outlook desktop client or an iCloud mailbox.

Email was designed to carry text. A PDF is binary, so before it can travel it gets re-written into printable characters using Base64, and Base64 is not free. RFC 2045 §6.8 puts it plainly: “the encoded data are consistently only about 33 percent larger than the unencoded data.”

The real figure is a little worse, because the same section requires that “the encoded output stream must be represented in lines of no more than 76 characters each.” Do the arithmetic: 76 Base64 characters carry 57 bytes of your PDF, and each of those lines ships with a carriage return and a line feed. So 57 bytes on disk become 78 bytes on the wire. That is 36.8% of pure overhead, which is exactly why Google’s own documentation says about a 37% increase rather than 33%.

Microsoft says the same thing from the administrator’s side. Its Exchange message size limits guidance tells admins to set the number roughly a third high on purpose: “if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size of approximately 48 MB.”

Now put the two halves together, because this is the failure nobody explains. Your sending client usually counts the file on disk. The receiving server almost always counts the encoded message. Attach a 24 MB PDF in Gmail and Gmail accepts it, because 24 is under 25. It leaves as roughly a 33 MB message. It arrives at a mailbox with a 25 MB receive limit, and it is rejected.

The file never changed size. The measurement did.

Divide the cap by 1.37 and round down. That is the whole method, and it is the number you should be aiming your compression preset at.

If you only remember one line from this page, make it the last row. When you do not know what is on the other end, 7 MB is the size that gets delivered. A CV, a signed contract, a 40-page report with a handful of figures: all of these compress well under that without anyone noticing.

Smol’s five PDF presets (tiny, small, medium, large, original) exist for exactly this decision, and the app shows you the resulting size per file before you send anything. Small is the email preset. Large is the one you want when the PDF is going to a printer and the file size is someone else’s problem.

Three routes, in increasing order of control. Our guide to compressing a PDF on Mac walks through every method in full; this is the short version for when you have an email open and a bounce message in front of you.

Preview. Open the PDF, File → Export, set Quartz Filter to Reduce File Size. It is free and it is already installed. It is also a single fixed recipe with no controls: we read the filter definition at /System/Library/Filters/Reduce File Size.qfilter on macOS 27.0, and it re-encodes every image as JPEG at quality 0.70, resamples to 144 DPI, and caps the longest edge at 2,400 px. Whatever your document is, that is what it gets. Sometimes that is fine. Sometimes it is far too aggressive for a scan you need to stay readable, and on one of our test files it produced a PDF nearly twice the size of the input.

Ghostscript. If you have Homebrew, brew install ghostscript then:

gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook \
   -dNOPAUSE -dQUIET -dBATCH \
   -sOutputFile=small.pdf original.pdf

On our test file (a four-page letter-size scan at 200 DPI, 1,596,556 bytes) that command returned 117,859 bytes, a 92.6% reduction, on an M2 Pro running macOS 27.0 with Ghostscript 10.07.0. Swap /ebook for /screen if you need to go further and the document is screen-only.

Smol. Drop the PDF in, pick a preset, read the output size, send. The reason we built the preset ladder rather than one button is that “small enough to email” and “still legible when the recipient prints it” are different targets, and you should get to choose which one you are optimizing for. Nothing uploads, which matters more than usual here: the PDFs people compress for email are contracts, medical records, and tax documents.

Whichever route you take, check the output size before you attach it. PDF compression is not monotonic. A tool can and does return a larger file than it was given.

Two different failures wear the same phrasing, and telling them apart saves you from compressing a file that was never the problem.

If your own client refuses before sending, you hit a client-side limit: the 20 MB or 25 MB ceiling in the table above, measured on the file. Compression fixes this.

If the message sends and a bounce arrives minutes later, a server rejected it. RFC 3463 defines the two codes you will see in the bounce. X.3.4 is “Message too big for system… The message is larger than per-message size limit”, a limit belonging to the whole mail system. X.2.3 is “Message length exceeds administrative limit… A per-mailbox administrative message length limit has been exceeded”, which means that one recipient has a stricter cap than their colleagues do. The second one is why the same attachment reaches four people at a company and bounces off the fifth.

The same arithmetic covers the other phrasings of this problem. “Photo too big to email” and “attachment too large” are the same encoding overhead applied to a different file type, and the same 7 MB safe target applies. For images specifically, format choice moves the number further than any quality slider, which we cover in the guide to compressing images on a Mac.

Send a link. This is the right answer more often than the internet admits, and all three major providers have built it in already.

Gmail does it without asking. Google’s documentation states that when your attachments exceed the limit, “Gmail automatically removes the attachment and adds it as a Google Drive link in the email.”

iCloud Mail has Mail Drop, which is genuinely generous and genuinely underused. Apple’s Mail Drop limits page puts it at attachments up to 5 GB, with recipients given 30 days to download, and the attachments do not count against your iCloud storage. It works from Mail on the Mac, from iPhone and iPad, and from iCloud.com.

Outlook.com raises the ceiling to 2 GB for files shared from OneDrive rather than attached.

The catch with all three is the same: the recipient needs to click out to a web service, which corporate firewalls and skeptical clients sometimes object to. If a real attachment is required, splitting a long PDF into two messages is less elegant and more reliable.

The PDF is already text-only and small. A 40-page contract with no images is typically a few hundred kilobytes. If that document is somehow 8 MB, the weight is embedded fonts or a scanned page hiding in the middle, and a compression preset is treating the symptom. Find the heavy page first.

The document is digitally signed. A signature covers the exact bytes of the file. Re-writing the PDF to make it smaller invalidates it. Compress before signing, never after.

It is going into a legal, medical or archival record. Resampling images in an evidentiary document changes the document. Send the original by link.

You do this twice a year. Preview’s Reduce File Size and Mail Drop are free, installed, and completely adequate for an occasional oversized attachment. We are not going to pretend otherwise. Smol earns its one-time $29 when this is a weekly problem, when you need a predictable target size rather than one fixed recipe, or when the files arrive in batches, which is a different job covered in batch compressing PDFs on a Mac.