There may be several reasons why letters in a line are not printed. Firstly, there is a difference between A4 or Letter size, which means that the PDF created on the printer set to A4 or Letter size is different with those of the printer.
Another cause may be in the left and right margins of both the PDF and those which are used in the printer.
The smartest solution is to ask the user to enable the "Adjust to page width" option in the chosen printer driver for an automatic adjustment, or otherwise use the hard way with try and error to select the "Scale" option to fit the print to fit.
There are so many different printers with so many (im)possibilities that a program solution for all those printers is not possible in practice. To help your users, you could generate a PDF that contains a plus sign on the left and right margins both above and below as a test page that uses little ink / toner to test the correct setting of the printer.