Explainer
JPEG, PNG or PDF: which format should you upload?
Choosing the wrong format is a rejection that has nothing to do with the quality of your document. Here is the short version.
The short answer
Photographs: JPEG. Almost always, and almost every portal says so explicitly.
Signatures: JPEG. Same reason, and the tight size limits make it more important.
Certificates, marksheets and multi-page documents: PDF, where the portal accepts it.
Screenshots, scanned text and line drawings: PNG is technically better, but many portals will not accept it.
Why JPEG is the default for photographs
JPEG is a lossy format. It discards detail that the human eye is unlikely to notice, which means a photograph can be made dramatically smaller with little visible cost. That is exactly what a portal with a 20 KB limit needs.
It is also universally supported, which is why it became the default for anything involving a camera.
The catch is that it is very bad at sharp edges. Text, line drawings and signatures get a halo of artefacts around each stroke, which is why a JPEG of a scanned document often looks muddy.
When PNG is actually better
PNG is lossless, so it preserves every pixel exactly. For screenshots, charts, logos and scanned text it produces a cleaner image than JPEG at a comparable visual quality.
It cannot discard detail to save space, though, so a PNG of a photograph is typically several times larger than the equivalent JPEG. That is usually fatal against a 20 KB ceiling.
Use PNG when the portal accepts it and the content is text or line art. Use JPEG everywhere else.
Why portals ask for PDF
PDF holds pages at a fixed size, keeps text as text rather than as pixels, and can bundle multiple pages into one file — which a marksheet or a certificate often needs.
It is also the format least likely to be altered unnoticed, which matters for documents being submitted as evidence.
A PDF of a scan is usually a scan wrapped in a PDF container. If the scan inside is a low-quality JPEG, the PDF will be small but the text will still be hard to read.
Converting between them
Converting a JPEG to PNG gains nothing. The detail JPEG discarded is gone, and PNG will faithfully preserve the artefacts while making the file larger.
Converting a PNG or a scan to JPEG is genuinely useful when you need to hit a size ceiling, which is the usual reason to do it.
Converting a JPEG to PDF is a reasonable way to submit a single-page document where the portal insists on PDF, and it is what most online converters actually do. It does not improve the image.
What to do if the portal wants a format you do not have
If you have a JPEG and the portal wants a PDF, wrapping it in a PDF is straightforward and lossless in practical terms.
If you have a PDF and the portal wants a JPEG, you will need to render a page of it to an image. This always loses some sharpness because you are turning text back into pixels.
If you have a PNG and the portal wants a JPEG, converting is safe but check the result: text will look softer than before, and if it looks bad, going back to the original scan and saving directly as JPEG is usually better than converting twice.
Questions people ask
Can I convert a PNG to JPEG without losing quality?
No — JPEG is lossy, so some quality is always lost. At high quality settings the loss is invisible for a photograph, but text and fine lines will look softer than in the PNG.
Why did my JPEG come out larger than the PNG?
Unusual, but possible when the image is mostly flat colour or line art, where PNG compresses very efficiently and JPEG does not. It is a hint that the content is not photographic.
Will the portal accept a HEIC file from my iPhone?
Rarely. HEIC is not universally supported. Convert to JPEG before uploading, and check the result because conversion can occasionally shift colours.
Does converting to PDF make the file smaller?
Not by itself. If the PDF contains a re-compressed image, the file may shrink, but that is the compression doing the work, not the container.
Which format should I use for a signature?
JPEG, because of the tight size limits. A signature is mostly blank paper, so it compresses extremely well as JPEG and will usually land far under the ceiling.