You can find nearly every walk-through, cheats and general FAQ's and reviews for every game in every platform right here:
http://www.gamefaqs.com/
I loved Doom 3, hated the PDA idea. It takes me too away from the action. I mean, you are very concentrated at the screen, walking in tiny dark corridors with great ambient and sound effects and all eye candy you need, waiting for a monster to jump on you, when you suddenly find a god damn PDA, with e-mails and video/audio data from somebody else, who is dead, just to find the code for a locker. Really bad planned. That's what I liked about Doom-classic, only action. If you have to do something telling a story, Half-Life did a way better job too. No reading, just listening and shooting.