Optimize & secure guide
Compress a PDF Under 10 MB for Email
Updated
Short answer: Run the file through Compress PDF on Medium, check the new size, and step up to High only if it is still over your mail provider's limit.
Email limits catch people out because the size your computer shows is not the size the mail server measures. Here is how to get under the limit on the first try, and what to do with a PDF that refuses to shrink.
Step by step
Step 1: Check the file's size (Properties on Windows, Get Info on a Mac) and the limit you're working to: 25 MB for Gmail, 20 MB for Outlook.com, and often 10 MB for company mail.
Step 2: Open Compress PDF, drop the file and leave the level on Medium.
Open Compress PDF(runs on our server)Step 3: Download the result and compare. If it's under about three quarters of the limit, you're done.
Step 4: Still too big? Run the original again on High, which shrinks images further.
Open Compress PDF(runs on our server)Step 5: If a mostly-text PDF still won't fit, split it into two parts and send them in separate emails.
Open Split PDF(runs on your device)
Why this works, and what to watch for
Attachments don't travel as raw bytes. Email encodes them as base64 text, which makes them about 33% bigger in transit, and many servers apply their limit to the encoded message. A 10 MB limit therefore means a file of roughly 7 MB, and Gmail's 25 MB means about 18 MB once you allow for encoding and the message itself. Gmail offers to send anything larger as a Drive link; most company servers simply bounce it.
Compression works because images are almost always what makes a PDF big. Compress PDF re-encodes embedded photos and scans as JPEG and scales down any that are larger than the level allows: Medium caps the longest edge at 1,600 pixels at JPEG quality 68, which is about 190 DPI across an A4 page; High caps it at 1,100 pixels at quality 48. Fonts, text and vector drawings are left alone, so text stays sharp and searchable. Unused objects are dropped and the internal streams repacked as well.
That's also why some files barely shrink. A report exported from Word is mostly text and fonts, which are already compact. Images with transparency, masks or inverted colours are skipped on purpose, because re-encoding them as JPEG would visibly damage the page. If the output wouldn't be smaller than the input, you get your original back rather than a bigger file.
Server tools accept uploads up to 10 MB on the free plan, so compressing a 40 MB scan needs Pro (50 MB) or Business (200 MB). The other route is to split the file first with Split PDF, which runs on your device with no size cap, and compress each part separately.