Quick answer
PDF files become too large when they contain scanned pages, embedded photos, high-resolution exports, design elements, or many pages. A scan can look like a simple document but still behave like a stack of large images.
Start with the number the portal gives you. If it says 200KB, use a PDF 200KB target. If it says 500KB, use 500KB. If it says 1MB, use 1024KB as the closest exact target.
What usually works best
If the document becomes hard to read, do not keep lowering quality blindly. Try a less aggressive target if the portal allows it, or use a sharper profile when readability matters more than the smallest possible file.
If the upload still fails after the PDF is under the size limit, check the accepted format, page count, password protection, and whether the portal rejects image-based scans.
If it still fails
A reliable order is: reduce the PDF to the stated limit, verify readability, then adjust PDF profile, render scale, or quality cap only if the site still refuses the file.