Keep an untouched original
Save a copy before applying compression. Give the smaller version a distinct filename so you can compare it with the source and return to the original if text or images become damaged. Repeatedly compressing an already reduced copy can compound quality loss.
Check the actual upload limit and the recipient's requirements. A portal may specify file size, page count or accepted formats. Aim to meet those requirements with some room to spare rather than shrinking the document as aggressively as possible.
For confidential documents, use an approved local tool or a service whose data handling is suitable for the material. Uploading a file to a convenient compression website gives that service the document, even if your eventual recipient is someone else.
Find the part that makes it large
A PDF created from a text document may be relatively compact until high-resolution photographs are added. A scanned PDF, by contrast, can contain a full-page image for every page. The appropriate compression settings depend on which kind of file you have.
If you still have the source document, reduce unnecessarily large images there and export again. That often gives more control than trying to repair the final PDF. Do not remove detail that the recipient needs, such as small labels in a diagram.
For scans, consider whether color is necessary. A document containing colored annotations or stamps may need it; a plain typed page may not. Choose deliberately rather than converting everything to black and white and discovering that useful distinctions disappeared.
Adobe's PDF optimization tools provide ways to adjust file size, but the result still needs a visual check. A tool's smaller output is not proof that every page remains suitable for its intended use.
Check the difficult pages first
Open the reduced file at ordinary reading size, then inspect small text, fine lines, signatures and detailed images. Compare those areas with the original. Look for fuzzy letters, blocky edges or faint marks that have vanished.
Search for a word and try selecting text if the document is meant to remain searchable. Compression should not be confused with OCR, which creates a text layer for a scan. Check links, forms and bookmarks if those are part of the file's purpose.
Print a representative page if the recipient will print it and the details are important. A screen preview can conceal problems that become obvious on paper.
Send the right version
Verify the final size after saving and closing the file. Open the exact copy you will attach, not an earlier version left in another folder. Use a name that makes the document and revision clear.
If meeting the size limit would make the file unreadable, ask whether a secure sharing link or a split submission is accepted. Splitting should follow the recipient's instructions, especially when page order matters. The successful outcome is a document they can read and use, not merely an upload that completes.
