What to check when your attachment hits the size limit
5 min read
What you do right after seeing “attachment too large” decides how the next hour goes. Checked in the right order, most cases resolve on the first try.
The real limit is smaller than the advertised one
A mail service's “25MB limit” applies to the whole message, not the attachment. Attachments are encoded for transport and grow by roughly 30% in the process, so the file you can actually attach tops out around 18–19MB.
That's why a 24MB file bounces off a 25MB limit. If your file is more than about three quarters of the stated limit, plan for another route from the start.
The recipient's limit may be smaller still
Even when your send succeeds, the receiving server can refuse the message. Corporate and institutional mail servers commonly cap incoming mail at around 10MB, and the bounce notice may not arrive for minutes or hours.
If you're sending important documents right before a deadline, anything over 10MB deserves either a quick question to the recipient about their limit, or shrinking before you send.
Decide whether the file can shrink at all
PDF compression results vary enormously by document type. Scans and photo-heavy files can drop below half size; a text-first document exported from a word processor may only lose a few percent.
Try compressing once. If the reduction is small, running the same file through again with different settings won't meaningfully change the outcome — switch to splitting instead.
When you split, split for the reader
Cutting a file mechanically by size can break a chapter — or a table — across two files. Split along content boundaries instead: by chapter, or by document type, and mark the filenames “1of2”, “2of2” so missing pieces are obvious.
State the total number of files in the mail body. When one message of a multi-part send lands in a spam folder, that line is the only thing that catches it.
When a link beats an attachment
For files of many tens of megabytes, uploading to cloud storage and sending a link is easier for everyone. But check submission rules first — some institutions only accept actual attachments for contracts and official filings.
If you do send a link, check the sharing permissions. “Anyone with the link” is convenient; for sensitive documents, granting access to the recipient's account specifically is the correct default.