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.
Practical example workflow
Prepare a separate sharing copy before editing a PDF. Keep the original file private, then create a review copy for compression, merging, redaction, OCR, or upload-size adjustments.
Open the final PDF in a different viewer from the one that created it. Check page order, rotated pages, missing fonts, links, signatures, stamps, metadata, bookmarks, and whether text can be searched or copied when that matters.
Quality checks before sending a PDF
If the PDF contains personal, financial, medical, school, or client information, remove unnecessary pages and use real redaction instead of covering text visually. Search for redacted words after saving to confirm they are gone.
For official submissions, keep a full-quality archive copy and a smaller upload copy with clear filenames. If a portal has file-size or format rules, check those rules before the deadline rather than compressing repeatedly at the last minute.