HP LaserJet 4050 Printer series - Manual (Page 65)

EN
Chapter 2: Printing Tasks 43
Paper Orientation for Printing Both Sides of Paper
The duplexer prints the second side of paper first, so paper such as
letterhead and preprinted paper needs to be oriented as shown in
Figure 9.
Figure 9 Paper Orientation for Printing Both Sides of Paper
For Tray 1, load the first side facing down and the top toward you.
For all other trays, load the first side facing up and the top toward
the printer.