What 100KB really buys you
100KB is among the strictest caps a document portal imposes, and whether it is generous or brutal depends entirely on what is in the document. A single-page certificate is mostly blank paper, and blank areas cost almost nothing to encode, so such a page fits at good resolution with room to spare.
The arithmetic changes fast with page count. The budget is shared across the whole document, so a five-page submission is really five pages at 20KB each — and 20KB per page of dense text is where legibility starts to be a genuine question rather than a formality. If your document is long and the cap is 100KB, the page count is the problem, not the compression setting.
Scan settings decide this before you compress anything
The single biggest factor in whether a document fits is how it was captured. A phone photograph of a page carries uneven lighting, a shadow from the phone, keystone distortion from not being square on, and visible paper texture. None of that is information anyone wants, and all of it is detail the encoder must spend your budget reproducing.
Scan rather than photograph where you can, and if you must use a phone, use its document mode — it flattens the perspective, crops to the page edge and normalises the lighting to white, which typically halves the size needed for the same readability. Scanning at 200 or 300 DPI is plenty; scanning at 600 produces a much larger file that compression then has to throw away again.
Greyscale rather than colour for ordinary text roughly halves the cost, because colour information on a monochrome page carries nothing but the paper's tint and the room's lighting. Keep colour only where it is evidence: an official stamp, a coloured signature, a security feature or an embedded photograph.
Checking the result properly
File size is not the test — readability is. Open the compressed PDF, zoom to the smallest text on the page, and read it. Registration numbers, dates and the fine print at the bottom of a certificate are exactly what a verifying clerk will look at, and exactly what degrades first.
If that text has gone soft, the fix is almost never a different compression setting. It is fewer pages in the file, a better scan, or greyscale instead of colour — all of which give the encoder more budget per page rather than asking it to do more with less.
Common reasons a 100KB upload is rejected anyway
Meeting the size limit is only one of several rules, and it is rarely the one that fails an application. Portals commonly also specify an exact page count, a particular file name format, a maximum number of pages per upload slot, or that a document be self-attested with a signature and date visible. A file that satisfies the size rule and breaks one of those is rejected just as firmly.
The other frequent cause is uploading the right document to the wrong slot. Application forms often have separate boxes with separate limits — commonly a tighter cap for a photograph or signature and a looser one for documents — and it is easy to compress a certificate down to a photograph's limit unnecessarily, or to submit a document against a slot that was never meant for it. Read the limit attached to the specific box before deciding what to aim for.