A PDF on a second monitor is not the same as the physical object. You cannot "feel" the page of a SimCity 2000 manual that explains how to zone industrial sectors. You cannot smell the cheap, pulpy paper of a Doom shareware manual. You cannot experience the thrill of unfolding a massive cloth map of the Betrayal at Krondor world.
DOS games had no such consistency. Every developer used different keys. The manual was your tutorial. dos game manuals
Because screens were low-resolution (320x200), there was no room for a HUD (Heads-Up Display). All the lore, stats, and key bindings lived on paper. You played with the manual propped open against your monitor, greasy pizza fingerprints accumulating on the "Combat" chapter. Before CD-ROMs allowed for voice acting and cinematic cutscenes, developers had two ways to build a world: pixel art and prose. A PDF on a second monitor is not