Compress a PDF for email attachment limits

Attachment caps are not suggestions. Gmail’s documented limit is 25 MB per message (the file plus MIME overhead — budget ~23 MB for the PDF itself). Outlook.com and Microsoft 365’s default is 20 MB for many tenants; Exchange admins sometimes set 10 MB. Common HR and legal portals (Workday uploads, court e-file, visa sites) sit at 5 MB or 10 MB. “The email bounced” is usually one of those numbers, not your ISP.

Worked sizes

42.6 MB lease scan, 28 pages. You need it under 10 MB for a portal, under 20 MB for Outlook, under 23 MB for Gmail. Recommended is the first try: that class of file often lands 6–9 MB, which clears all three. If it lands at 12 MB, you are fine for Gmail and still over the portal — run High, accept that pages become images, or split into two emails.

18.2 MB board deck with photos. Recommended often 8–11 MB. High will flatten charts; do not do that if someone still needs to copy a number from a slide. Gmail will take 18.2 MB anyway — compress only if a recipient’s tenant is the 10 MB kind.

184 KB Word export. Do not compress. You are already under every cap. High will make it worse.

Overhead you forget

Base64 in transit adds ~33%. A 20.0 MB PDF can become a 26.6 MB SMTP payload and bounce on a 25 MB server. If you are hugging the cap, aim 20% under it. Several files in one mail share one cap — merge first only if the combined size still fits; otherwise send two messages or a link you control.

None of this requires uploading the packet to a compressor. The file is already the thing you did not want on someone else’s disk.