Hi MrJglass,
The potential issue with studying from these types of documents is that the exact 'source' of the questions might not be clear - and working through those questions can lead to 'inflated' results later on.
For example, if you had a document that included all of the 'hardest' questions from the Official GMAC practice CATs (but the document didn't clearly state that fact) - and you worked through those questions BEFORE taking the Official CATs 3, 4, 5 and 6, then you would know the correct answers to lots of the most difficult questions in advance - and THAT knowledge would completely 'throw off' the Scoring Algorithm, your CAT results and impact your pacing, energy levels, fatigue, etc.
The potential harm could be severe - you would think that you were scoring at a much higher level than you actually were, you'd go into Test Day not properly prepared AND you'd end up working through a bunch of practice CATs that were not useful to you. Considering how important a high GMAT Score is - and the amount of time and effort that most Test Takers would need to spend to earn that type of Score - the risk of working through these types of documents is far too high to be considered a good idea.
GMAT assassins aren't born, they're made,
Rich