Page size other than default and print

All my pages in my document are 8x10 inches, as this is going to be a 8x10 book. Now first of all, this definitely means, when I click “export directly as pdf”, it will automatically export it as a 8x10 inches document right? So I can submit this to the publishing company or amazon self publishing etc… like this, and it will be a 8x10 document? Please answer this first as this is the bottom line that matters for me.

Second question: I also ask because when I first tried to print it on paper from libre writer, the margins don’t seem to add up. Yes I had 8.5x11 paper in my printer tray, and lets say it aligned things to upper left corner when printing, but still the bottom margin, when I measure doesn’t add up. It left definitely more space on printed paper than my bottom margin plus the footer height than I see on my screen even when I account for 10 inches vs 11 inches paper. I also tried from printing from pdf version, and still the same problem. What might be the cause and how can I get it to print exactly?

Create a document with those paper sizes and export to PDF. Open the file with a PDF reader and look at the properties for page size. That will answer your first question.

The second part of the question seems to relate to how your printer driver handles a mismatch between page size and paper size. Remember most printers now ask for size of paper loaded or detect it