FASA Trek worked by setting skills on a 1-100 percentile scale. Games like the old RuneQuest were similar. FASA also graded a characters competency. Certain task might not require a roll depending on the character's skill.
For example if piloting a shuttlecraft under routine conditions, anyone with a skill rating of 10 or higher was considered a qualified shuttle pilot and would not need to make a skill roll. Those with skills of 1-9 would need to make a skill roll on 1D10 against their skill. Those without the skill at all would be in a tough spot and would have to hope for some successful LUC rolls to avoid a mishap.
The system really didn't work in much finer detail though. While the 1-100 range was (and still is) much larger than the 1-6 scale in ICON, almost all skill rolls were of the succeed/fail variety. I don't even think it had rules for degrees of success and failure, so it actually played with less detail than ICON.
Overall, the game had it's good points and bad points. It's biggest drawback being that it was created and expanding in during a tie when Trek was not very active. So when the films took off and Next Gen got started, it led to some differences between the FASA setting and official Trek.
Yes, there probably are PDFs for FASA out there on the net, but we cannot recommended you to download them, tell you where to locate them, or how, as this would violate copywrite and product licenses and could get people and this board into trouble. Sorry.
We can say that the books do pop up regularly at places like eBay, and in the used bin at many local gaming shops. And at good prices.
And, of course we can discuss and and all facets of the game with you.