Find the real limit
Some portals list a maximum file size per file, while others limit the total upload packet. Check whether the limit is measured in MB or MiB and whether multiple files are allowed.
If the portal rejects encrypted files, scanned images, or unusual PDF versions, compression alone will not solve the issue.
Reduce the right source of size
Scanned pages and embedded photos usually create the largest PDFs. Cropping, grayscale conversion, and balanced image compression can reduce the file without removing required pages.
If a PDF is made from Word, Excel, or Slides, fix the source export settings before using heavy compression.
Check before submitting
Open the final upload copy and verify page order, signatures, stamps, small text, and bookmarks. A smaller file is not useful if the reviewer cannot read it.
Keep a full-quality archive copy and a smaller submission copy with clear filenames.