JPEG or PNG: which scan format protects stamps and signatures?

A fax with a blue stamp, a signed authority form, a colour exhibit from a government portal: three sources, three risks. In firm practice the image format quietly decides whether OCR will hold and whether someone must rescan before a portal upload.

JPEG and PNG are not a matter of taste. They govern file size, edge artefacts, and the readability of fine lines. Optimising only for “as small as possible” saves megabytes and spends them later on rework.

What JPEG does well in practice

JPEG suits photographic exhibits: site photos, accident scenes, equipment images. Lossy compression keeps the file manageable if contrast and sharpness remain adequate. For text pages with even grey and clear type, a moderate JPEG quality can also work - for example for internal working copies that will be re-typeset anyway.

JPEG becomes risky once stamps, signatures, handwritten marginalia, or fine table lines matter. Typical artefacts appear as mush along edges. OCR then reads “8” for “B”, matter references break, and a second zoom review costs more than the larger source file would have.

When PNG is the better choice

PNG stores losslessly. For black-and-white and greyscale pleadings with stamps, seals, and signatures it is often the more robust base - especially if OCR, cropping, or contrast correction follow. Files grow larger, but they stay technically honest: what was scanned remains visible.

Many firms combine approaches deliberately: PNG or high-quality TIFF as the archive and OCR source, compressed PDF only at the end of the chain. That separates quality control from transmission size. LexLogik supports browser workflows where source quality is not spent in the first save.

A simple decision rule for teams

Define which document classes require which format. A workable standard:

• Signatures, stamps, seals, handwritten additions: PNG (or equivalent lossless).
• Clean printed text without critical micro-marks, internal second copy: JPEG at a documented quality level.
• Mixed packs: treat critical pages separately; do not force one setting through everything.
• Before portal upload: check critical pages at 150-200% zoom - edges, digits, stamp borders.

Document the quality level (not only “high/low”) and who releases the file. Otherwise every cover arrangement invents a new quasi-standard.

Common mistakes before deadlines

Just before upload, teams often “compress once more”. That is when artefacts appear that thumbnails hide. A second pair of eyes on stamp and signature pages is cheaper than a rejected portal attempt.

Keep the original scan separate from the compressed filing copy. If readability is later disputed, you need the lossless source - not only the last PDF that went out.

Release before portal upload

Have a second person zoom-check stamp and signature pages before the compressed filing copy is produced. Support notes quality level and format in the matter line; the matter lead releases when deadlines or agency portals are involved. The format rule then stays part of the deadline chain, not an IT footnote.

Once format, resolution and release are fixed, you avoid rescans under deadline pressure. Choosing JPEG or PNG belongs in the same team standard as filenames and OCR language - not in whoever touched the scanner that day.