You might want to try changing the String SubData Type on that attribute to being HTML within Object Designer. I expect it will be found under Knowledge Management - Article - Solution. Try it out on your test system and see if this works as expected.
As an aside, you can build in the contents of .pdfs into the knowledge search itself if you are licenced for active knowledge. If you are on 7.4 and 7.5 then you get this as standard.