There is 64 PDAs (If you count your own), 32 Storage Lockers and 17 videos in the DOOM 3 campaign, You have to collect all of them for collectible trophies. There is also 9 PDAs (If you don't count your own) in the Lost Mission campaign, there is no need to collect the Video Logs or open the lockers in this campaign as there is no trophy for doing so. Finally there is 22 PDAs in the Resurrection of Evil campaign, like Lost Mission you only need to collect the PDAs as there isn't any trophies for the Video Logs or Lockers. Here are the trophies you will unlock by following this guide:
Collect every PDA in DOOM 3
I Like to Watch
Find all video logs in DOOM 3
Open all storage lockers in DOOM 3
Collect every PDA in the RoE campaign
Collect every PDA in the Lost Mission campaign
Q. I missed a PDA(or other collectible), what do I do?!
A. Well, you are screwed for getting the achievement. You cannot backtrack through levels and there is no level select option, so if you miss one then you will have to start again. What I recommend doing is saving your save on a new slot every time you transfer to a new level. That way if you miss one you can go all the way back to the one you missed. Sadly, you'll have to re-collect all of them past that point.
Q. Can I check how many I have?
A. You can check on your pda, press and it will tell you all the names of the PDAs you have and also video logs. You cannot check lockers however.
Q. I unlocked Goody Finder early
A. This is a known glitch, but it's a good glitch. It might be because of the Martian lockers, in the videos below I only opened 1 of them (2nd one in Delta Labs). You will unlock the achievement by opening only the storage lockers, it just unlocks early if you open the others. I haven't included the others in this guide.
Q. If I die and have to reload my save from before, do I have to pick them up again from where I reloaded?
A. Yes you do, but I'm sure this won't be too much of a problem as you should be saving every so often