From what have seen this question has been popping up for years, and there's no answer. Maybe I can find one here. I'm running OS X 10.11.6 (although as far as I can tell this problems exists in all recent versions of hardware and software) on an iMac 7,1,, and the most recent Mac version of Adobe Reader includes the option to "choose paper source by PDF size." That option had been missing in the Mac version for years, all the while being in the Windows version and working well, as long as you had your printer properly set up for dual-tray dual-paper size printing. However there was no way to get that printer to do automatic dual-tray dual-size printing on a Mac. That option existed nowhere. Then finally the option was added in one of the recent updates to Adobe for the Mac. The problem is that whether or not you check that option, it is ignored and you STILL cannot dual-print automatically. Interestingly, when I run Windows 10 in Parallels on the same iMac, and use the Windows version of Adobe the checkbox DOES work and I can do dual-printing on that same printer attached to the iMac. So the question is, why? Why if the option to 'chose paper source by PDF size' is now included in Adobe Reader for the Mac does it not work? Can it be made to work? (To be clear, I'm NOT talking about scaling, where you get a 8x14 page to squeeze onto an 8x11 page. I'm talking about having the printer switch from Tray 1 [holding 8x11] to Tray 2 [holding 11x14] automatically to feed the correct page size.) Thanks for any help. P.S. The printer must be properly configured, because as I said above the dual-printing works through Windows, and even on the Mac I can control which paper tray to use from the printer driver manually. But there is no option for automatic source tray selection in the OS X printer driver and, as I said above, the Mac Adobe command to do so is ignored.