A smaller PDF is useful only if its text and pictures remain usable. Compression settings should follow the purpose of the document.
Read the upload rules
Check the maximum file size, page count and allowed format on the receiving website. A limit stated in KB can use a different byte convention; leave a small margin. Do not remove pages required by the application simply to reach a size target.
Choose a balanced first pass
Use Compress PDF and select a moderate level. Image-heavy scans usually shrink more than PDFs made mostly of text and vectors. Stronger settings may downsample images or change their compression. Already optimised files may not shrink; retain the original if a result is larger.
Inspect small details
Reopen the result and zoom in on identification numbers, stamps, tables and faint handwriting. Look at pages with photographs and fine linework. If details disappear, return to a higher quality setting or reduce unnecessary scan resolution at the source.
Prepare the final upload
Rename the exported file clearly and verify its actual size. If the portal still rejects it, check whether the PDF is encrypted, damaged or above a page limit. Compressing repeatedly can degrade a scan without providing much extra saving.