Where a 500KB limit comes up
Half a megabyte is a frequent limit on university application systems, bank and insurance document uploads, loan and rental applications and many web forms. It suits documents of a few pages, such as a transcript, a payslip or a bank statement.
The target is already set to 500KB. If the form says 0.5 MB, keep it; if it says 512 KB, you can type that instead.
What to expect at 500KB
Digital PDFs with photos or charts usually fit with only a small loss in image quality. Text, fonts and links are not touched, so the document still looks crisp when printed.
Scanned documents of three to five pages in grayscale usually fit as well. Larger color scans may be turned into page images at a lower resolution; the result row then tells you that text can no longer be selected. If the target cannot be reached at all, you get the smallest version and a note that the target was missed.
When the file is still too big
Check whether the form accepts several files. Splitting a long statement into parts with Split PDF keeps each part sharper than squeezing everything into one file. For new scans, 150 to 200 DPI in grayscale is plenty for typed documents.