There's something quite amusing about the fact that the Doctor with the least number of appearances has the second largest book so far.
I honestly think the Sixth and Seventh Doctors could have benefitted from the approach here with some original adventures created for the books rather than stretching their smaller number of stories across the same page count as the earlier incarnations.
Can someone advise me on how I could sign up for the "advanced copys". I collect my RPG in PDF format only and am curios where I would go to get these.
I'm afraid you can't now. The offer which included these was only open for a limited period in advance of the 1st Doctor sourcebook being released back in early 2013 - it was a sort of thank you to subscribers for stumping up their cash in full in advance.
There was also a Later Doctors subscription covering the 7th-11th Doctor sourcebooks which I guess also included advance pdfs. But again this offer was time limited and closed before the release of the 7th Doctor book last year.