dougwilson - 3:29 am on Mar 1, 2013 (gmt 0)
I have a few pdf files on my site - if people aren't on my site (external requests for .pdf) I have an "answer this question" form that must be passed before files can be read, downloaded etc.
I was redirecting using the no hot link rule [F], but I like this better.
Aside from the scrapers riding up the serp on the backs of other people's content I had literally thousands of 404's from malformed external links.
Still have them since Google still crawls all the pdf scraper sites. Also use no index but I always check several (other) pages at redbot dot org after using these rules to make sure there is no weird inheriting going on.