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What DPI should you export a PDF at?
PDF pages are vector instructions with no resolution of their own, so exporting to images means choosing one. Here is how to pick, and why 300 DPI is usually wrong for screens.
A PDF page has no resolution
This is the thing to understand first, because every other question follows from it.
A PDF page is a set of drawing instructions in an abstract coordinate space, put this glyph here, stroke this line from here to here. Text and vectors have no pixel dimensions at all. The page has a size, measured in points (1/72 of an inch), but it has no resolution until something renders it.
So "export at the original resolution" is not a request that can be honoured. Exporting to PNG or JPEG means choosing a resolution, and the only question is which.
The exception is a scanned document, where the page really is one big image at a fixed resolution. Rendering that above its native resolution enlarges pixels and gains nothing: see below.
The arithmetic
A page is 8.5 × 11 inches. Multiply by the DPI you choose:
| DPI | Pixels | Use |
|---|---|---|
| 72 | 612 × 792 | A thumbnail. Body text is unreadable. |
| 150 | 1275 × 1650 | Fine on screen, readable, modest file. |
| 200 | 1700 × 2200 | Comfortable for reading on a laptop. |
| 300 | 2550 × 3300 | Print. Also what OCR wants. |
| 600 | 5100 × 6600 | Fine-line artwork for print. Rarely otherwise. |
The file size does not scale with DPI, it scales with the square of it. Doubling 150 to 300 quadruples the pixel count.
300 is a printing number, not a screen number
"Use 300 DPI" is advice inherited from prepress, where it is correct: it is roughly where an inked halftone stops being resolvable by eye at reading distance.
For a page that will be looked at on a screen it is usually two to four times more than anyone will see, and the cost is real: a 25-page document at 300 DPI is tens of megabytes of PNG.
Pick from the destination instead:
- Posting an image of a page: 150 works, 200 is generous.
- Reading on a laptop or tablet: 150 to 200.
- Printing it again: 300.
- Feeding it to OCR: 300. Text recognition genuinely does better with
more pixels per character, and this is the one case where going higher than the screen needs is not waste.
- Fine line art or small print for print: 600.
PNG or JPEG
PNG is lossless. Text and line art stay crisp with no artefacts around the edges. It is the right default for documents, which are mostly text on white.
JPEG is lossy and compresses photographs far better. For a page that is mostly photographs it can be several times smaller with no visible difference; for a page of text it produces coloured fringes and mush around letterforms and is often larger than PNG, because sharp black-on-white edges are exactly what its compression handles worst.
Rule of thumb: text pages PNG, photo pages JPEG, mixed pages PNG unless the size is a problem.
Upscaling a scan gains nothing
If the PDF is a scan, the page contains an image at whatever resolution the scanner produced: often 200 or 300 DPI. Rendering that page at 600 does not recover detail that was never captured. It interpolates, producing a file four times the size, slightly softer, containing exactly the same information.
Work out what the scan actually is before choosing. Exporting at or slightly below its native resolution is the honest maximum.
Why a big document takes a while
Each page has to be fully rendered, fonts resolved, vectors rasterised, embedded images decoded and scaled, and then encoded. At 300 DPI that is around 8 megapixels a page.
Done in the browser, this happens on your machine and nothing is uploaded, which is the trade: a hundred-page document takes a minute or two of your own CPU rather than seconds of somebody's server and a copy of your document on their disk.