Choosing a resolution for PDF to PNG: 96, 150 or 300 dpi
A PDF page is described in physical units, not pixels. To turn it into a PNG, the page has to be drawn onto a grid of pixels — and the dpi setting decides how fine that grid is.
What dpi decides
dpi (dots per inch) sets how many pixels each inch of the page gets. An A4 page is 8.27 inches wide, so at 150 dpi it becomes about 1240 pixels wide. Double the dpi and the image gets twice as many pixels across and four times as many in total.
| Page | 96 dpi | 150 dpi | 300 dpi |
|---|---|---|---|
| A4 | 793 × 1123 px | 1240 × 1754 px | 2479 × 3508 px |
| US Letter | 816 × 1056 px | 1275 × 1650 px | 2550 × 3300 px |
In the settings panel, Compact, Standard and High each show the pixel size of your own first selected page before you save.
A starting point for each use
| You want to… | Start with |
|---|---|
| Show the page on a screen, in a slide or a chat | 96 or 150 |
| Keep a readable copy of a text page | 150 (the default) |
| Zoom into small print, thin lines or a diagram | 300 |
| Print the image at the page’s original size | 300 |
These are starting points. If text looks soft when you zoom in, go up a step; if the file is larger than you need, go down one.
Why PNG grows faster than you might expect
PNG stores the pixels without lossy compression, so the file size follows the pixel count and the amount of detail. Going from 150 to 300 dpi multiplies the pixels by four. In our tests with the same A4 pages:
- a page of text went from about 0.5 MB to about 1.2 MB,
- a page filled with a detailed photograph went from about 3.5–4 MB to about 11.5–13 MB.
The exact numbers vary with the content and the browser. That is why the settings panel adds a reminder when you choose High: it does not stop you, it tells you that the files can be large. PNG file size and what “lossless” means goes into this.
More dpi does not add detail
A scan made at 100 dpi does not become sharper when it is drawn at 300 dpi: exporting a low-resolution scan at a higher dpi does not bring back detail that is not in the source. Higher resolution helps with content the PDF describes precisely — text, lines and vector drawings — and with photos that were embedded at high resolution.
When the number you chose is not the number you get
To keep memory in check, a single page is limited to 8192 pixels on its longest side and 24 megapixels in total. A normal A4 or Letter page stays well below this even at 300 dpi. Very long, very wide or very large pages can reach it. Our wide test page (2380 × 595 pt, about 84 × 21 cm) went past the limit when 300 dpi was chosen and was scaled down, with its proportions kept, to 8192 × 2048 pixels.
When that happens, the page is scaled down with its proportions kept. Before you save, a note under the page selection tells you that a selected page will be scaled down and names the lowest resolution that will actually be used; the settings panel shows the same note next to the pixel size.
Convert your pages in the browser → PDF to PNG