PDA

From Doom Wiki

The PDA is one of Doom 3's most useful items. It acts as a media player, email reader and access key for restricted areas.

Besides the player's personal PDA, other PDAs can be picked up and scanned to read helpful hints (including backstory, hints to monster types ahead, and combination codes for lockers).

[edit] Secrets

The last level contains the id PDA which contains special messages from the developers of the game. Immediately before entering the room with the Cyberdemon, there is a button marked with the id Software logo. Pressing this opens a door to a secret room containing the PDA.