So, the accumulated PageRank of all three pages is 36/23 which is just over half the value that we could have expected if page A had links to one of the other pages. According to Page and Brin, the number of dangling links in Google's index is fairly high. A reason therefore is that many linked pages are not indexed by Google, for example because indexing is disallowed by a robots.txt file.
Additionally, Google meanwhile indexes several file types and not HTML only. PDF or Word files do not really have outbound links and, hence, dangling links could have major impacts on Page ranking algorithm. I wouldn't worry about your PR, its not a ranking factor and so thinking about it so much is not needed. For example, have a look at some pdf sharing sites and you will see those work just fine and link to many pdf's. I wouldn't advice blocking google from the pdf's because that will cut down the amount of pages and content your site has, so no I wouldn't nofollow them nor place them in the robots file personally.