Comment by tcas

As someone who's dealt with these protocols as well I agree.

I find the print quality of PDFs perfect on Zebra's as long as you keep the following in mind:

  - Make sure the DPI of the document is correct (e.g. 203dpi)
  - Make sure the document does not need to be scaled (and scaling to fit is disabled) (for 4x6 that means 812 x 1218 pixels max)
  - Make sure the document is black and white (no grayscale)

ZPL is a beast.

Some PDFs embed a single image in them, in which case you can do the following;

  pdfimages -all <input_file>.pdf <out_prefix>
  convert <generated_png>.png -filter point -density 203 -units PixelsPerInch out.png

And then print the generated png.