HP LaserJet Enterprise 600 Printer M603 series - Manual (Page 94)

Load paper trays
Load Tray 1
NOTE: The product might print at a slower speed when using Tray 1.
CAUTION: To avoid jams, do not load trays while the product is printing. Do not fan the paper.
Fanning can cause misfeeds.
1. Open Tray 1.
2. Pull out the tray extension.
78 Chapter 6 Paper and print media ENWW