Check the whole message
Add up the size of all attachments. Email encoding can increase the transmitted size, so staying exactly at a stated file limit may not be enough. Confirm whether separate attachments or shared links are accepted.
Start with optimization
Try structure optimization first. An already compact PDF may not shrink; when there is no gain, the tool keeps the original. Compare the result with the size permitted by your destination.
Consider strong compression carefully
Strong compression creates image-based pages and removes selectable text, links, forms and verifiable signatures. Start with balanced quality and inspect small letters, values and codes. If readability suffers, ask the recipient about another delivery method instead of deleting required information.