CalcLockAt a glance: Choose the format that keeps the required content readable within the destination rules. PNG is useful for crisp graphics; JPEG can be practical for photographed pages and strict size limits.
Start with what the destination accepts
Check the receiving site's or person's requirements before choosing a format. Some forms accept only JPEG, while others accept PNG or prefer a PDF for multipage material. Follow the actual requirement rather than a general rule that one format is always better.
If you are keeping a personal source archive, preserve the original capture or scan as well as any submission copy. A file prepared for a small upload limit may be unsuitable for later printing or closer inspection. Name the purpose of the copy so you do not mistake it for your best available source.
Look at the image content
A screenshot with sharp black text and flat white areas differs from a photograph of a wrinkled receipt under uneven light. PNG uses lossless compression and supports transparency; JPEG commonly uses lossy compression and does not preserve an alpha transparency channel. Those differences affect practical choices, not the truth or completeness of the document.
For crisp screenshots, a PNG is often a sensible starting point. For photographic scans with shading and texture, a JPEG may fit a small file limit more easily. Compare actual files rather than assuming that every page behaves like the last one.
Test the smallest meaningful details
Create candidate exports from the same source and compare them at the same scale. Examine small digits, thin punctuation, signatures, and lightly printed text. Compression damage that is invisible in a page-sized preview may make a decimal point or a reference number ambiguous.
Do not choose solely by the smallest byte count. A 300 KB file that obscures the amount on a receipt is worse for that task than a larger acceptable copy. Equally, a very large PNG offers little benefit if it merely preserves an already blurred photograph without improving legibility.
Handle transparency deliberately
A transparent background can be useful for a logo but is usually unnecessary for a document page. If the source contains transparent areas, decide what background the recipient should see. Dark text that looks good against your viewer's white canvas may become difficult to read against another background.
For a conventional page image, create a deliberate opaque background when appropriate and verify the exported result. Do not rely on whichever background color your current viewer happens to display. Check edges around text and stamps so flattening has not introduced unexpected halos or missing content.
Export a new file and inspect it
On a Mac, Preview offers File > Export with format choices including JPEG and PNG. Save the candidate with a new name in your preparation folder. For JPEG, adjust quality only as far as the task allows; keep the untouched source available for another attempt.
Reopen the saved file and inspect it independently of the export dialog. Confirm the extension matches the actual format, the orientation is right, and the dimensions remain useful. Converting an existing damaged JPEG to PNG does not restore lost detail; it preserves the pixels you currently have.
Remember what a format choice does not solve
Neither JPEG nor PNG automatically removes metadata, private text, or information visible in the background. Both can reproduce a document that you should not disclose. Review the exact outgoing content and destination after choosing the format.
An image of text is also not the same as an accessible text document. If the recipient needs searchable text, screen-reader access, or several ordered pages, a properly prepared document may be a better deliverable. Do not assume a PNG or JPEG export includes text recognition merely because you can read the page on screen.
Questions, answered
Does PNG make a blurry document sharper?
No. Lossless compression avoids an additional lossy encoding step, but it does not recover detail missing from the source. If small text is unreadable, improve the capture or obtain a better source before converting formats.
Should every scan become a JPEG?
No. Follow destination requirements and compare the actual result. A text screenshot, a photographed page, and a multipage record may call for different formats.