The attachment bounced, or the upload form refused the file.
Your PDF is too big to email. Here's what's actually in it.
Most oversized PDFs are oversized for one of three reasons, and each has a different fix. Guessing wastes a round trip with the person waiting for the file.
First, work out which kind of big it is
A PDF is a container. The same 40 MB can be one badly scanned page or four hundred clean ones, and the two need opposite treatments.
Open the file and look at a page closely. If the text is selectable, the weight is almost certainly images placed alongside it — photos, logos, or a scan sitting under a text layer. If nothing is selectable, every page is a picture, and the page count barely matters compared to the resolution each page was captured at.
Common mail services cap attachments around 20–25 MB, and many upload forms are stricter. That is the number you are trying to get under, not some abstract idea of small.
If the pages are scans, compress before you do anything else
A phone camera photographs a document at far more detail than reading it requires. Compression re-encodes those images at a level you choose, which is why the same tool can take a file from unusable to fine without touching the words.
Compress at a gentle level first and look at the result. If the text is still crisp, you are done. Push harder only if you have to, and check the pages that matter — small print, stamps, signatures — because those degrade first.
If the recipient only needs part of it, send part of it
A 300-page report where someone asked about section four does not need to be sent whole. Extracting the pages they asked for is faster than compressing, and the result reads better on their end.
This is also the honest fix when compression is not enough. Two emails with clean pages beat one email with unreadable ones.
Check the result before you send it
Compare the file size against the limit you are actually trying to clear, not against how much smaller it got. A 60 percent reduction is irrelevant if the result is still 30 MB.
Then open the compressed copy and read the pages that carry the weight of the document — the signature, the figures, the small print at the bottom of the terms. Compression degrades detail unevenly, and a page of body text can look perfect while a stamp turns to mush.
Keep the original. If the compressed version turns out to be too degraded for whoever receives it, you want to be able to try a gentler setting rather than re-scan the paperwork.
What not to do
Do not rename a PDF to a different extension to sneak past a filter — the recipient will not be able to open it, and mail scanners usually catch it anyway.
Do not re-scan at a lower quality without checking the original first. If a good scan already exists, compressing it keeps more detail than re-capturing it badly.
Do not convert pages to images and rebuild a PDF from them to save space. That destroys any text layer, so the file stops being searchable and stops being accessible to a screen reader.