PNG quality and PDF file size
PNG is a lossless format: saving a PNG doesn’t throw away detail the way saving a JPG does, which is why screenshots, charts and diagrams are usually PNG. Here is what PNG to PDF really stores.
There are two options in Page setup: Quality first, the default, and Smaller file. Details under them gives a short version of this guide.
Quality first: lossless, never JPEG
With Quality first, the pixels of every PNG are stored in the PDF with lossless compression (Flate, the PDF name for the same Deflate method that PNG uses). No image is converted to JPEG, with either option.
- 8-bit PNGs — colour, palette, greyscale and interlaced — keep their exact pixels in Chromium-based browsers such as Chrome and Edge. We measured a difference of zero.
- 16-bit PNGs are decoded by the browser to 8 bits per channel, and that is what goes into the PDF. The largest difference we measured was 1 level out of 255 in Chromium and 3 in Safari’s engine, WebKit. It looks the same, but it is not a bit-for-bit copy of the 16-bit file.
- Transparency is kept as a separate soft mask — exact in Chromium-based browsers, at most 1 invisible level off in Safari for greyscale or 16-bit PNGs; PNG transparency in PDF explains why it still looks white on the page.
A note on Safari
In Safari, greyscale PNGs come out with tiny deviations — up to 2 or 3 levels out of 255, too small to see — and are stored as colour instead of grey. That can make the PDF noticeably larger: a 400 × 300 grey gradient took 2 KB in Chromium and 151 KB in WebKit. If you have many greyscale PNGs and size matters, a Chromium-based browser makes the smaller PDF.
Very large images
Your browser decodes every image, and there is a device limit: images above 16,777,216 pixels (4096 × 4096) are scaled down to fit it — a 6000 × 4000 image, 24 megapixels, becomes 5016 × 3344. Its card says Scaled down to fit the device limit, and after saving the result strip counts such images, for example 1 image scaled down.
Most screenshots are far below the limit. A 1170 × 12000 tall screenshot (14 megapixels) and a 12000 × 1000 panorama (12 megapixels) both go in at full size.
Smaller file
| Image | Quality first | Smaller file |
|---|---|---|
| Long side ≤ 2000 px | unchanged | unchanged |
| Long side > 2000 px | full size* | long side 2000 px† |
| Long screenshot or panorama | full size* | short side at least 600 px† |
| Compression | lossless | lossless |
| Transparency | soft mask | soft mask |
* Images above the device limit are scaled down with both options.
† Only if that makes the image smaller in bytes; otherwise it keeps its Quality first version, pixel size included.
For every image longer than 2000 px on its long side, Smaller file prepares a candidate: normally its long side becomes 2000 px, still lossless, transparency still kept. The candidate is used only when it really is smaller than the Quality first version. Images of 2000 px or less are not touched at all, and no image is ever enlarged. How much your own PDF shrinks depends entirely on your images — there is no fixed percentage, and it may not shrink at all.
Long screenshots and panoramas
Bringing the long side down to 2000 px would make a long image very thin: a 1170 × 12000 scrolling screenshot would become 195 × 2000, and its text could no longer be read. So when that rule would take the short side below 600 px, Smaller file keeps the short side at 600 px and scales the long side in proportion. An image whose short side is already under 600 px is neither enlarged nor made smaller; it keeps its Quality first version. When this happens, the strip below the images says that the long image’s readability was preserved.
In our tests in Chrome, a 6000 × 4000 image became 2000 × 1333 and its PDF went from 175 KB to 58 KB. A 1170 × 12000 screenshot became 600 × 6154, its PDF went from 404 KB to 104 KB, and its text could still be read. A 12000 × 1000 panorama became 7200 × 600, and its PDF went from 122 KB to 65 KB.
Why a PNG PDF is as big as it is
The size of the PDF comes almost entirely from the image data, stored losslessly. Screenshots and charts, with large areas of flat colour, compress well, so a PDF of them is often small. A photo saved as PNG takes many bytes to store losslessly, and the PDF stays large; here, the only way to shrink it is Smaller file.
See the real size
After you tap Save PDF, the strip at the bottom shows the number of pages, the real file size and any images scaled down. To compare, save once with Quality first, switch to Smaller file and save again.
Which one to choose
- Keep Quality first for screenshots with small print, charts you will zoom into or print, and anything you archive.
- Choose Smaller file when large images — over 2000 px — meet a size limit, and losing pixels on those images is acceptable. Long screenshots keep at least 600 px on their short side, so ordinary text stays readable; very small print can still become hard to read.