The short answer
When quality has to stay exactly as it is, split rather than compress. Splitting by size fills each part to just under the limit, so you send as few attachments as possible and every page is untouched.
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Step by step
- Work out the real limit Gmail and most Microsoft 365 mailboxes stop at 25 MB, company gateways often at 10 MB. Attachments grow by about a third in transit, so target 18 MB for a 25 MB cap.
- Decide between splitting and compressing If the document is a scan and quality has some room, compressing to one file is kinder to the recipient. If it is a print-ready or archival document, split it.
- Split by size, not by page count Pages vary enormously — one photograph can outweigh forty pages of text. Splitting by size packs each part properly instead of producing one huge part and three tiny ones.
- Name the parts so they are obviously a set Rename to "Report 1 of 3", "2 of 3". A recipient who gets three unnamed attachments will ask which is which.
- Say so in the email One line explaining that the document is in three parts and should be read in order saves a reply asking.
Questions
Should I zip the parts instead?
Zipping barely shrinks a PDF, because its streams are already compressed. It is useful for bundling several files into one attachment, not for getting under a limit.
Will splitting change the pages?
No. Splitting copies pages into new files without re-encoding anything, so every part is exactly as the original was.
Can the recipient put it back together?
Yes, by merging the parts in order — which is the same tool in reverse, and produces the original document.
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