The usual 200 KB uploads
Admission and recruitment forms often allow around 200 KB per document for things like a class 10 or 12 mark sheet, a degree certificate, a caste or income certificate, or an Aadhaar or PAN card copy. That is enough room for a clear, readable copy, provided the scan is not much larger than it needs to be.
Photos of documents
A phone photo of a certificate is often 3 to 5 MB as a JPG. Put it into a PDF with Images to PDF first (crop it to the paper), then compress the PDF here. Or, if the form accepts images, compress the photo directly with the image compressor.
Several documents, one upload
When a form wants all certificates in a single PDF under 200 KB, merge them first, then compress: the compressor shares the budget across the pages. Greyscale helps most here.