Portals check six things: file size, page count, encryption, MIME type, structural validity, and page dimensions. If you rule out each in order, one of them explains the rejection. When in doubt, run the file through a repair pass and strip anything the portal might not like.
Upload portals take files from strangers, so anything unexpected is rejected automatically. The error message stays vague on purpose, because a detailed rejection reason helps an attacker probe the system. That leaves you guessing, and the six checks below are the guesses worth making first.
The file is too big
This is the most common rejection. Portals print a size limit in or near the upload widget, and a file over it is refused at once. The limit differs from portal to portal and changes over time, so read the one on the form in front of you rather than a list. USCIS online filing, for example, takes evidence files of up to 12MB each in PDF, JPG or JPEG, while some exam and job portals set their limits in kilobytes.
The fix is compression aimed at the portal's stated size. The target pages step down through progressively stronger settings and keep the first result that fits: compress to 100KB, 200KB, 500KB, 1MB, 2MB or 5MB.
The file has too many pages
Some portals enforce a maximum page count regardless of file size. Court e-filing systems and academic submission portals often limit pages per document, and an ID upload slot usually expects the front and back of a card, not a ten-page scan. The limit, when there is one, is normally stated in the filing instructions rather than next to the button.
Fix by splitting the file. Use Split PDF to break the source into portal-sized chunks and upload as separate submissions.
The file is encrypted
Many portals reject password-protected PDFs even if the user provides the password. The portal system needs to inspect the content for indexing, virus scanning, or search, and encrypted files block all of that. The upload succeeds visually, then the server-side scan fails and the file is bounced from the queue.
The bytes do not match the extension
Portals often verify that the file's bytes match a real PDF signature. Files renamed from other formats (a Word document saved with a .pdf extension, an image with a swapped name) fail this check even though they look correct in a file browser.
%PDF at position 0. Open the file in a plain-text editor. If the first four characters are anything else, it is not a PDF. Re-export from the original source, or open it in a real PDF viewer and save as PDF from there.The PDF structure is slightly broken
PDFs are databases of interconnected objects. Slight corruption, from an interrupted save or an out-of-spec exporter, can leave the file readable in your reader but unacceptable to a strict validator. Portal validators tend to be strict.
Repair rebuilds the header, cross-reference table and catalog from whatever objects survived, and writes a new file. A file your reader opens but a validator refuses is the case it handles best. Run it through Repair PDF and upload the rebuilt copy before you go back to the sender.
The page dimensions are wrong
This one is rare. Some portals enforce a paper size (A4 or Letter) and reject anything else. Scanner output and design exports often have non-standard pages, and a merged file can hold several sizes at once, which is its own problem covered in fixing a PDF with mixed page sizes.
Fix it by putting every page on one standard sheet. Resize PDF places each page on A4, Letter or another size with the content scaled to fit and centred, so nothing is cropped. Printing to PDF with the paper size set explicitly and "Fit to page" selected does the same job, but can drop links and form fields on the way.
The five-minute diagnostic
Run through the checks in this order. The first two take seconds and need no tool.
Check the size
File Manager or Finder shows the byte count. If it is over the portal's cap, compress the PDF first.
Check page count
Open the PDF, look at the page counter. If over the cap, split.
Check for password
If the file opened without prompting for a password but the portal still refuses it, run through Unlock PDF anyway to strip owner-password restrictions.
Check the header bytes
Text editor, first four characters. If anything other than %PDF, re-export from source.
Run repair
If nothing above applies, repair-and-resave produces a clean structural copy.
Normalize page size
If still rejected and the portal has a paper-size hint, print to PDF at that specific size.
The "boring PDF" recipe
For the strictest portals (government submissions, court e-filing, medical uploads), make a plain copy that carries nothing a validator could object to. Work in this order, on a copy, because flattening cannot be undone.
First remove any password or restriction with Unlock PDF, since the later steps need to read the file. Then flatten form fields and annotations into the page with Flatten PDF, and clear the author, creator and title fields with the metadata remover.
Compress to just under the portal's stated cap, and finish with a pass through Repair PDF so the file you upload has a freshly written structure.
Compress a PDF to the portal's exact limit
Pick a level, or use a target page for 100KB, 200KB, 500KB, 1MB or 5MB that steps down until the file fits. Runs inside the browser.
Common questions
Details that come up before hitting Submit again.
01Why does the portal reject my PDF without saying why?
Portals rarely explain rejections in detail because a detailed error would help attackers probe the system. Six causes account for most rejections: size, page count, encryption, damaged structure, wrong MIME type, and page dimensions. Ruling them out one at a time is faster than trying random fixes.
02What is a MIME type problem?
Some portals verify that the file's bytes match its .pdf extension. If the file was renamed from another format, or the extension was set by hand, the byte-level check fails. Fix by re-exporting from the original or by opening the file in a real PDF viewer and saving as PDF from there.
03Do government portals ban encrypted PDFs?
Many do, because their virus scanning, indexing and archival systems cannot read an encrypted file. The rejection can arrive after the upload looked successful, when the server-side scan fails. If a portal's own instructions ask you to password-protect a file, follow those instead and send the password the way they specify.
04What is the safest way to prepare a PDF for a strict portal?
Remove encryption, flatten, strip metadata, compress, then repair, and do it on a copy. Flattening turns form fields and comments into part of the page for good, so the uploaded file is a one-way export. Keep the original for any later corrections.
05Can page dimensions cause a rejection?
Occasionally. A4 and Letter are not the same shape as well as not the same size, so a page fitted from one to the other gains a thin margin on two sides. That is normal and portals accept it; what they refuse is a page that is neither.
06Does PDFslime upload files during any of this?
No. Every tool mentioned here runs in your browser, and the file is never sent to a server. That matters for the identity documents and financial records portals usually ask for.