PDF to JPG: which resolution?
What resolution should I use when converting a PDF to images?
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Short answer
150 DPI for anything that will be looked at on a screen, 300 for anything that will be printed or read closely, 72 only for a thumbnail. A PDF page has no resolution of its own — it is a description, and the number you choose is what decides how many pixels get drawn from it.
Pick a number
Converting a page to an image means deciding how finely to sample a description that has no grain of its own.
What resolution actually decides
- 72 DPI. Fine on a screen, visibly stepped on paper.
- 150 DPI. Acceptable for an office printer and a document nobody will study.
- 300 DPI. What a commercial printer expects, measured at the size the image is actually printed.
| DPI | An A4 page becomes | Use it for |
|---|---|---|
| 72 | about 595 x 842 | Thumbnails, previews |
| 150 | about 1240 x 1754 | Screen reading, email, most web use |
| 300 | about 2480 x 3508 | Printing, archiving, anything read closely |
| 600 | about 4960 x 7016 | Rarely worth it outside fine detail work |
Doubling the DPI roughly quadruples the pixels — and the file size. 600 is not "twice as good" as 300; it is four times the bytes for a difference most eyes cannot find.
JPG or PNG
Decided by what is on the page, not by which is newer.
Photographs and screenshots want opposite formats
- A photograph: thousands of subtle gradations, no hard edges. JPG is built for this.
- A screenshot: flat colour and sharp boundaries. Nothing here is subtle.
- JPG on an edge. It stores an approximation, so the boundary smears into the pixels beside it.
- PNG on the same edge. Every pixel is kept, so the boundary stays exactly where it was.
JPG for pages that are mostly photographs. Small files, and the approximation it makes is invisible in a photo.
PNG for pages that are mostly text, diagrams, tables or screenshots. It keeps every pixel, so hard edges stay hard — which is precisely what text is made of.
A page of text converted to JPG at 150 DPI is the commonest way to get a result that looks faintly dirty: JPG puts a halo around every letter, and a page has thousands of them. JPG or PNG for a PDF? covers the same decision in the other direction.
Keeping the result manageable
Three hundred pages at 300 DPI is a great many megabytes, and most of the time nobody needed that.
- Convert only the pages you need. Extract pages first.
- Match the number to the destination. A picture going into a slide does not need print resolution.
- Do not convert at all if you can avoid it. If the goal is a smaller PDF, compress it — turning the pages into images makes the text unsearchable and usually makes the file bigger.
Going back the other way
JPG to PDF puts the images back into a document, and it is worth being clear about what you get: a PDF containing pictures of pages.
The round trip is lossy in a way that does not undo. Text that went in selectable comes back as pixels, search finds nothing, and screen readers get a blank page. If you need the text back, that is OCR, with the recognition errors that implies.
So convert a PDF to images when images are what you want — a slide, a thumbnail, a preview. When you want a smaller or a different document, stay in PDF.
Tools this article covers
Sources
Primary documentation for the claims above.