A PDA (or Personal Digital Assistant) is an electronics device that one can carry around and use for day-to-day planning and sometimes includes internet access. PDAs have nowadays been largely replaced by Smartphones.
The first PDAs appeared in the early 1990s like the Palm Pilot. There were never any Tomb Raider Games released for any of the many PDAs that appeared throughout the 1990s and 2000s.
Tomb Raider Legend
The PDA Lara carries in Legend takes over most functions of the Inventory. Lara can not only check out her gear there, but she can also store and read messages and mission details. Descriptions of the gear she carries are displayed, as well as the layout of Croft Manor. Lara also uses the PDA like a diary, recording her thoughts of a particular mission.
The PDA is waterproof and can even accompany Lara on her long journeys through water.
Lara carries it in Underworld as well. It now includes an active sonar map; a revolutionary new tool that emits a sonar ping to create a 3D image of Lara's surroundings, perfect for uncovering hidden items and locations. 
- http://support.na.square-enix.com/document/manual/1015/Tomb_Raider_Legend_XBOX360.pdf Legend Booklet
- Fact Sheet by Eidos