DPI explained: choosing a resolution for PDF to image
What DPI means when converting a PDF to JPG or PNG, how to work out pixel size, which DPI to use for screens, OCR and print, and why browsers cap very large pages.
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A PDF page has no pixels
Text and vector graphics in a PDF are stored as shapes and outlines, measured in points — 72 to an inch. They only become pixels when something draws them: your screen, a printer, or a converter. DPI (dots per inch, strictly PPI for images) is simply how many pixels you ask for per inch of paper.
So the pixel size of an exported page is page width in inches × DPI. US Letter is 8.5 × 11 in, so at 150 DPI it becomes 1,275 × 1,650 px; A4 (8.27 × 11.69 in) becomes 1,240 × 1,754 px.
A page that only turns into pixels when something draws it is a neat puzzle; for another tidy one, try ahaboo’s narrated explainer on why the Moon has phases.
Which DPI for which job
| DPI | Letter page in pixels | Good for |
|---|---|---|
| 72 | 612 × 792 | Thumbnails, quick previews |
| 150 | 1,275 × 1,650 | Email, chat, web uploads, reading on a phone |
| 200 | 1,700 × 2,200 | Sharp small print, OCR |
| 300 | 2,550 × 3,300 | Printing at actual size |
| 600 | 5,100 × 6,600 | Archiving, fine line art (capped in browsers — see below) |
300 DPI is the long-standing print-shop target because at normal reading distance the eye can’t resolve finer detail. Going higher mostly makes files bigger.
Why scanned PDFs don’t get sharper at higher DPI
If a page is a scan, it already is pixels — say a 200 DPI scan from an office copier. Rendering it at 600 DPI just enlarges those pixels. To get the scan’s real resolution without resampling, use Extract images, which saves the stored image as-is.
The browser’s canvas limit
Browsers draw PDF pages on a canvas, and canvases have a maximum size. iPhone and iPad Safari refuse anything above about 16.7 million pixels; desktop browsers allow more but slow down badly. pdfimage.app keeps every page under 16 megapixels: an A4 or Letter page is fine up to about 470 DPI; above that, or for big posters and drawings, the page is rendered at the highest DPI that fits and the result shows the actual DPI used.
DPI when going from images to PDF
The opposite direction has the same maths. A 4,032 × 3,024 px phone photo placed on an A4 page with a small margin prints at roughly 380 pixels per inch — more than enough. A 480 px web image stretched across the same page drops to about 45 pixels per inch and will look soft; the “Don’t enlarge small images” option on the image to PDF converter keeps it at its natural size instead.
Questions
Is DPI the same as PPI?
Strictly, PPI (pixels per inch) describes images and DPI (dots per inch) describes printers. Converters and most people use DPI for both, and so does this site.
Does changing DPI change the page size?
Only in pixels. The image still describes the same physical page; print it at 300 DPI and a 2,550 px-wide image comes out 8.5 inches wide.
What DPI do government or visa forms want?
Commonly 200–300 DPI for scanned documents, often with a file-size limit. Check the form’s instructions; 200 DPI JPG at 85% quality is a good compromise.