Select page size using document page size seems to be over-written by Paper size:

 * Xreader version (xreader --version)
xreader 2.6.4
 * Distribution - (Mint 17.2, Arch, Fedora 25, etc...)
Mint 20 Cinnamon 4.6.7

Issue
the "Paper size" field on the Page Setup tab overrides the checkbox "Select page size using document page size" on the Page Handling tab.

Steps to reproduce
Print a document which contains multiple page sizes, such as letter and legal, to a printer with multiple trays containing appropriate sizes, such as letter and legal.

When letter is selected from the Paper size field all pages print to the paper tray with letter size paper regardless of the size page in the document. This has the effect of cutting off the bottom of the page for legal size pages in the document.

When legal is selected from the Paper size field all pages print to the paper tray with legal size paper regardless of the size page in the document. This has the effect of leaving blank space at the bottom of the letter size pages in the document.

Expected behaviour
Printer should print from tray with appropriate size as designated by the document with out reference to the Paper size drop drown.

Other information
Printer: Brother HL-L6200DW series with 2 trays, one letter, one legal.

Start by reproducing the issue in Xreader 2.6.4 on Mint 20 with a document containing letter and legal pages and separate printer trays. Trace how the Paper size field interacts with the document page-size checkbox; done means each page selects the tray matching its document size regardless of the Paper size selection.