Great suggestion, thanks a lot! At first, this tool refused to work saying
```
Image with transparency and a bit depth of 16.
This is unsupported due to PIL limitations.
error: Refusing to work with multiple >8bit channels.
```
however I didn't need any transparency here, so I:
- opened the original PNG image in Gimp;
- picked a suitable background color;
- added a new layer filled with this color;
- merged the original layer into the new background layer;
- removed the alpha channel from the resulting layer (right-click on the layer → Remove Alpha Channel);
- used File → Export As... to write a new PNG file without the alpha channel.
After this, `img2pdf` worked quite fine on the new file. The sizes of the original PNG and the resulting PDF file are very close (the PDF file being slightly smaller: 17,023,902 vs. 17,050,961 bytes).
Besides, your link taught me the existence of the `pdfimages` tool which is fantastic to:
- list all images embedded in a PDF file (`pdfimages -list foo.pdf`);
- extract them, e.g. with `pdfimages -all foo.pdf someprefix` (there are options to choose output formats, `-all` being equivalent to `-png -tiff -j -jp2 -jbig2 -ccitt` according to the `--help` output).
:-)