There has been a lot of talk here about the page orientation selection option not available during printing.
Today, I created a portrait letter size poster. When I tried to print it out within Writer [5.3.5 on Ubuntu], the thumbnail of the page showed a landscape page with the right half showing the document as if there was 2 pages per sheet of paper.
I even made sure the default of the printer driver was portrait.
I had no problem Exporting to PDF and printed the PDF file as a portrait document.
Then I came a solution, by accident.
I changed from the Canon MP6220 to my Canon TS9020 printer, then back again. "holy keyboard, Batman" It changed the printer orientation from landscape to portrait.
So, maybe this "fix" works with other systems. Give it a try.
Since I am running Ubuntu 16.04 and LibreOffice 5.3.5, I hope it was fixed in 5.3.6. I will be installing 5.3.6 today, maybe, on this system. I will see what that version works like over the current version.