GMail now has an internal PDF reader so you don't have to "View in HTML" or "Download" and then use the usually-resource-heavy Acrobat Reader to view them. This could have a significant impact on Reader's install base. Of course, it won't make PDF's any less useful. Also - I doubt Gmail's reader will let you fill out interactive forms, etc.
Still, it is a big change. That is, unless you use Mac's "Preview," which opens like a breeze. Durn Mayc Usehrs! [with apologies to my schitzo mac mini.]