You have a PDF file with page layout as A4, you want it to be printed on Letter paper. However it always printed to A4 instead of staying on letter. In this case, check the following;
In Foxit Reader Print dialog box, see if the option “Choose Paper Source by PDF Page Size” was checked, if so, uncheck it. With this option enabled, Foxit Reader will pick up a paper size that is most matched with the current PDF page layout, and ignore the printer settings.
When you try to print a word document into a PDF file with the Foxit Reader PDF printer, the generated PDF file page size needs to be set in the print dialog box of the word application itself and the layout setting in Foxit Reader PDF printer dialog box won't take effect.
For MS word 2007, please choose the page size in “zoom/scale to paper size" option in word print dialog box.
For MS word 2010, please choose the page size in "settings/page sizes" option in word print dialog box.
For MS Word 2013, choose Page Setup under the Settings in Word print dialog box.