PDF toolsdpi means dots per inch: how many pixels are used for each inch of the page. A PDF page has a physical size (A4 is 8.27 × 11.69 inches), so the resolution decides how many pixels wide the resulting image is.
The numbers for common pages
| Resolution | A4 (pixels) | US Letter (pixels) | Good for |
|---|---|---|---|
| 72 dpi | 595 × 842 | 612 × 792 | Thumbnails, previews |
| 150 dpi | 1240 × 1754 | 1275 × 1650 | Screens, email, social posts |
| 200 dpi | 1654 × 2339 | 1700 × 2200 | Zooming in, small print |
| 300 dpi | 2480 × 3508 | 2550 × 3300 | Printing |
Doubling the resolution doubles the width and the height, so the image has four times as many pixels and is roughly four times as heavy.
Which to choose
- On screen, 150 dpi. A typical laptop screen shows a full A4 page about 1000 pixels tall. 150 dpi gives you enough to zoom a little.
- For printing, 300 dpi. That's the standard for print. Going higher rarely shows on paper.
- For small text, 200 dpi. Footnotes in 7 pt type can look soft at 150 dpi when people zoom in.
- Never 72 dpi for text you want to read. It's fine for a thumbnail, poor for anything else.
Why 72?
PDF measures everything in points, and there are 72 points in an inch. Converting at 72 dpi means one point becomes one pixel, which is why an A4 page at 72 dpi is 595 pixels wide: that's its width in points. The number also echoes early Macintosh screens, which showed 72 pixels per inch, so a document on screen matched its printed size.
Scanning
The same logic applies to scanners. For text documents, 200 to 300 dpi in greyscale is plenty, and 150 dpi is often enough if the scan will only be read on screen. Colour at 600 dpi is for photographs and artwork, and it produces very large files. More on that in why your PDF is so big.