Using PdfWriter, there is no way to get a sub-millimeter block size. Phones are capable of reading QRCode with block size of below 1/3 of a millimeter.

In order to get around this, I used that modified version of PdfWriter PdfWriter.txt (uploaded with .txt as .patch is blocked by github for some reason) but it feels a bit rough. Maybe there is a better way to unlink the block size to the output unit size.