These are small devices found throughout each level which offer some insight into the story behind Alien Swarm.
They show how some of the miners died, how they fought, how they tried to survive the Swarm and even their personalities and relationships. Produced by SynTek, these small devices hold entries which resemble personal logs. They appear as tablets with a lit screen and a single large green light. It can be assumed that 'PDA' stands for "Personal Digital Assistant" though they do not look like modern day PDA devices. A list of all unique employees is located at PDA Correspondents.
All PDAs are listed by map first and then by the order in which they appear.
Additional messages from the same people can be found in the Alien Swarm directory. If a map name is given, it is deducted from in-game appearance of other messages with the same filename.
Probably would have been used in a dedicated tutorial mission, instead the missions 1-3 of Jacob's Rest were patched together, thus these items stayed unused.
Probably would have been used for Deima Surface Bridge.
Probably would have been used for SynTek Residential.
Probably would have been used for Timor Station.
Probably would have been used for SynTek Labs.