PNG file size and what “lossless” means
PNG is a lossless image format: once a page has been drawn as pixels, those pixels are stored without change. That is why text edges and thin lines in a PNG show no compression artefacts — and also why some PNG files are surprisingly large.
What “lossless” covers
The PNG format compresses pixel data without discarding anything, using the same kind of compression as ZIP. Decode the file and you get back exactly the pixels that were encoded.
What lossless does not mean:
- It does not keep the PDF as a PDF. The page is drawn at the resolution you choose, and from then on it is a fixed grid of pixels. Zoom in far enough and you will see them.
- It does not add detail. A PNG at 96 dpi is lossless, but it holds far less detail than one at 300 dpi.
- It is not a copy of any one viewer’s screen. Fonts and fine lines are drawn by the tool’s own PDF engine and can look slightly different from another app.
In short: PNG loses nothing after the page has been drawn. The resolution you pick decides how much of the page’s detail is there in the first place.
Why size depends on the content
Lossless compression works well on large areas of the same colour and repeating patterns, and poorly on fine, irregular detail. In our tests at 300 dpi on A4 pages:
| Page content | PNG size |
|---|---|
| Text and thin lines | about 1.1–1.3 MB |
| Half photo, half text | about 4–4.4 MB |
| A detailed photograph filling the page | about 11.5–13 MB |
Each range shows the sizes we got from the same test page in two different browsers. In these samples the content made much more difference than the browser; your own pages will give other numbers.
Why size depends on resolution
Doubling the dpi means four times as many pixels. From 150 to 300 dpi, the photo page above grew about 3.3 times and the text page about 2.5 times. The settings panel shows the pixel size before you save and adds a reminder at High; it does not block it.
When JPG is the better choice
For scanned pages and photographs, JPG is usually much smaller. On the same full-page photograph at 300 dpi, the JPG was roughly 7 to 15 times smaller than the PNG, depending on the browser, and still looked fine on screen. For pages of text and line drawings, the two came out in a similar size range, and only the JPG showed compression artefacts around the letters.
If your pages are mostly photos or scans, PDF to JPG gives you smaller files. If they are text, charts, diagrams or screenshots where clean edges matter, PNG is a good fit.
Keeping files manageable
- Use Standard (150 dpi) for anything that will only be seen on a screen.
- Choose only the pages you need; saving several pages shows how.
- Keep High for pages you will zoom into or print.