Email apps and application portals still reject heavy PDFs. The compress tool redraws each page as a JPEG and builds a new PDF at the same page size. Start with Balanced. If the form names a size, use Max size. If you want the file about 40% smaller, use Smaller by %. If a diagram turns muddy, use Sharper and try again.
The trade is honest: text in the compressed file is no longer selectable, because the page is now a picture. Keep the original on your phone and send the compressed copy. If you only needed a lighter picture of one page, PDF to JPG may be a better fit.
Compression stops at 80 pages so the phone can finish. A longer scan should be split first. Stay on the page until the progress text reaches the last page and the download button appears. Switching apps mid-way can cause the mobile browser to pause the tab.
Compressing does not remove a password and it does not hide sensitive sentences. If a number must disappear, use Redact and cover it before you shrink the file.