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How to compress a PDF for email: real size limits and what to cut first
The email bounces, or Gmail turns your attachment into a Drive link. Before reaching for an online compressor, it helps to know what the limits really are and what makes a PDF heavy in the first place.
Gmail accepts up to 25 MB of attachments per message, and Microsoft lists 20 MB for Outlook.com and similar accounts (often 10 MB on company Exchange servers). Attachments grow about a third when sent, so aim for well under the limit: under 10 MB is a safe target. Big PDFs are almost always big because of images, so reduce image resolution first. Compressing rewrites the file, so don’t compress a signed PDF.
The real limits
| Service | Limit | What happens over it |
|---|---|---|
| Gmail (personal) | 25 MB in total per message | Gmail removes the attachment and adds a Google Drive link instead |
| Outlook.com and other internet accounts | 20 MB per message, per Microsoft | Error when attaching; Outlook suggests sharing a link |
| Exchange (work email) | 10 MB by default, set by your IT | Error or bounce |
| Upload portals | Often 2 to 5 MB | Upload refused |
Email encodes attachments as text (Base64), which makes them about 33% larger in transit, and the limit can apply to the encoded size. A 19 MB PDF can fail at a “25 MB” limit. That’s why under 10 MB is a comfortable target for email and under 2 MB for most portals.
Why PDFs get big
- Scanned pages at high resolution. A colour page at 600 dpi can be several megabytes. Text is perfectly readable at 200 to 300 dpi in grayscale.
- Photos placed at full camera size, shown small on the page.
- Old versions inside the file. PDFs saved incrementally carry every earlier version.
- Embedded fonts and attachments, usually a smaller share.
How to shrink a PDF
On a Mac, with Preview
Open the PDF, choose File › Export, and pick Reduce File Size in the Quartz Filter menu. Apple warns the result may be of lower quality, and the filter can’t be adjusted: text-heavy files barely shrink, while photos may turn blurry. Check the result before you send it.
Rescan smarter
If it’s a scan, rescanning in grayscale at 200 to 300 dpi often beats any compressor, and the text stays crisp.
Online compressors
They work, but they upload your document. Fine for a menu, not for a payslip or a medical report.
Don’t compress, share
For very large files, a shared link from iCloud Drive, Google Drive or OneDrive avoids the problem altogether.
Before you compress
- Signed PDFs: compression rewrites the file and invalidates digital signatures. Send signed documents as they are, or by link.
- Forms: some compressors flatten fields. If the recipient must fill the form, check it still works.
- Keep the original. Compress a copy.
- Clean up while you’re at it: remove your name from comments and the document’s properties if they don’t need to see it.
Keepsign’s Ready to send makes a copy for the place it’s going: Email (under 10 MB), Upload portal (under 2 MB) or Smallest, on iPhone, iPad and Mac, without uploading anything. It works on the images, keeps text and form fields intact, and in the same step can fix fields and notes onto the page and remove personal info. Your original stays exactly as it is, and if it’s signed, Keepsign tells you the copy won’t carry a valid signature. Removing personal info is free; compressing to a size is part of Pro. Scanning in Keepsign.
Sources
- Google, Gmail Help, “Attachment size limit”, support.google.com/mail/answer/6584
- Microsoft Support, “Send large files with Outlook”, support.microsoft.com (20 MB for internet accounts such as Outlook.com; 10 MB default for Exchange).
- Apple, Preview User Guide for Mac, “Reduce the file size of a PDF”, support.apple.com/guide/preview/prvw1509/mac
- RFC 2045, MIME Part One, section 6.8, Base64 content transfer encoding (4 output characters for every 3 input bytes).