I use adobe pdfcould depend on the quality settings of the pdf printer
What do you use?
Ah ok sure maybe it is so certain if you do it on an a4 who sees it you say.Could it depend that you are looking at the pdf with a magnification ratio 1600 % (i.e. 16 times larger) ?
i.e. print and fill an a4 with a rectangle originally of 1,3x1.8 cm?
above all set the print resolution (quality=custom, you activate the dpi box and manual writing 600)