The black background problem
A PNG can carry an alpha channel: pixels that are partly or fully see-through. A PDF page has no such thing to inherit from, so the converter has to decide what sits behind those pixels.
Many tools do the naive thing and draw the image onto a blank canvas. A blank HTML canvas is transparent black, and when that gets encoded to JPEG the transparency disappears and the black stays. That is why a transparent logo often lands in the PDF as a white shape on a black square, or as a dark halo around the edges.
This tool paints the canvas white before drawing. Transparent becomes white, half transparent becomes a lighter version of the colour, and the result matches what you saw on screen.
PNG is the wrong format to be converting from
Not always, but often enough to say plainly. PNG is lossless, which is exactly right for screenshots, logos, diagrams and anything with flat colour and text. It is a poor fit for photographs, where it produces files three to ten times larger than a JPEG nobody can tell apart.
| Content | PNG in | PDF out | Verdict |
|---|---|---|---|
| Phone screenshot | 1.8 MB | ~350 KB at High | Fine. Keep quality up. |
| Photograph saved as PNG | 12 MB | ~1.2 MB at High | Big win, no visible cost. |
| Logo on transparency | 90 KB | ~120 KB | Works, but a vector PDF would be better. |
| Scanned text page | 4 MB | ~600 KB at High | Readable. Do not go below High. |
Screenshots specifically
Screenshots are the single most common PNG people convert, and they are the hardest case for JPEG. JPEG throws away high frequency detail, and the edge of a letter is nothing but high frequency detail. That is where you get the faint grey mosquito noise around text.
At the High setting it is invisible at normal zoom. At Smallest it is obvious. If you are converting screenshots for a bug report or a receipt, keep it on High and accept the file size. There is a page dedicated to this at screenshot to PDF.
Sizing the page
Screenshots are an awkward shape for paper. A phone screenshot is roughly 9:19.5, so on A4 it fills the height and leaves wide white bars down both sides. That is correct behaviour, not a bug, and Fit the photo avoids it entirely if the PDF is only ever going to be read on a screen.