First rule: follow the form
If the form names one format, use that one. Portals often pass files straight into a case-management system that only understands certain types, and a file in the wrong format may be accepted by the upload box but fail later without anyone telling you. Also check the size limit and whether the field accepts several files or only one. Everything below applies when you genuinely have a choice.
What each format is good at
PDF is a document format. It can hold many pages in a fixed order, each with a defined paper size, and it can contain real text as well as images. It prints predictably at the right size. Reviewers open it in a viewer designed for reading documents.
JPG is a photo format. It stores one image with lossy compression tuned for photographs. It is universally supported and small for photographic content, but it has no concept of pages or paper size, and repeated saving degrades it.
PNG is a lossless image format. It keeps sharp edges perfectly, which suits screenshots and diagrams, but it produces large files for photographs.
Use a PDF when…
- The document has more than one page. A three-page contract as three JPGs can arrive in any order, or the reviewer may only open the first. One PDF keeps the pages together and in order. Use Image to PDF to combine phone photos into one file.
- The document was created digitally. A certificate or statement downloaded as a PDF should be uploaded as that PDF. It is sharp at any zoom, often smaller than an image, and its text can be searched and verified. Printing it and scanning it back, or taking a screenshot, only makes it worse.
- It might be printed. A PDF has a paper size. An image printed from a browser may come out tiny, cropped or stretched.
- Several documents go into one slot. Merge them into one PDF with Merge in the order the form lists them.
Use an image when…
- It is a photograph. A passport-style photo, a picture of damage for an insurance claim, a product photo. These are images by nature; wrapping them in a PDF adds nothing and some systems will not display a PDF as a photo.
- The form will show it back to you or others. Profile photos and ID thumbnails are displayed as images on the page.
- It is a single page and the form asks for a "scan" as JPG. Some systems crop and check ID images automatically and need an image file.
If you have a PDF and need an image, PDF to Image exports each page as PNG, JPG or WebP at a resolution you choose.
Signatures
If a form asks you to upload a signature, it usually wants a small image — often a PNG with a transparent or white background, sized a few hundred pixels wide. Sign in dark ink on white paper, photograph it flat in good light, and crop tightly. If you need to put your signature onto a PDF, Sign PDF places a PNG signature on the page in your browser. Note that an image of a signature is not the same as a cryptographic digital signature; if a form requires a certified e-signature, it will say so and provide its own system.
Size limits change the answer
When the limit is small, format matters more than people expect.
- For a photographic page, a JPG and a PDF containing that JPG are nearly the same size. The PDF wrapper adds only a few kilobytes.
- For a digital text document, the original PDF is usually far smaller than any image of it. A one-page statement might be 60 KB as PDF and 400 KB as a readable PNG.
- For a scan, the size depends on resolution and colour, not on the format. Reduce those; changing format alone rarely helps. The guide How to reduce a PDF to under 300 KB covers this.
Readability is the real test
A reviewer at a visa office or HR department may have a few minutes per applicant. Whatever format you choose, the reader should see the whole page the right way up, be able to read every number without zooming in, and not have to open five files to find one piece of information. Before uploading, open your file on a phone as well as a computer — some reviewers work on tablets.
File names
Use plain, descriptive file names without special characters: Lee_Jiwon_passport.pdf, not IMG_20260914_093412(1).jpg. Some portals reject spaces, accents or non-Latin characters, and the name is often the first thing the reviewer sees.
Decision summary
- Form specifies a format?Use it.
- More than one page, or a digital document?PDF.
- A photo, or a single image the form will display?JPG.
- Screenshot or diagram with fine lines?PNG, or PDF if it has several pages.
- Tight size limit?Reduce resolution and colour first; format second.