Firstly, welcome to
40k!
2) Chapter Approved is sort of an annual expansion / balance / something. The 2017 one contained extra missions, Relics and Strategies for the forces that were still using Index lists (this is now almost zero, so that won't in the 2018 one), as well as some rules changes (Character targetting, since errata'd again) and points costs changes. The 2018
CA will include a beta Codex for Sisters of Battle, and almost certainly more points cost changes, but beyond that I'm not sure.
3)
GW doesn't really do .pdf copies, you've most likely ended up with a dodgy copy. They do do electronic versions (epub format), and "enhanced" electronic versions, which sometimes are updated with cost changes and sometimes not.
1) The
GW FAQs can be found
HERE, Beta rules are found in the
BIG FAQs which are released twice a year. Beta rules are Errata that are not "finalised" - i.e. they have been published for people to try, and feed-back thoughts to
GW about.