It is interesting to think about history. If Adobe had done a little bit better job in its design of the Postscript language, HTML would never have existed. Everything would be written in PostScript. Unfortunately, the internal structure of Postscript was just too quirky for a mass audience. It is possible to learn the basics of writing HTML in a few short sessions. Even more important, it is possible for normal people to write web pages in notepad. Postscript just has too many oddities to capture the mass market.
Anyway, PostScript did not capture the mass market. Most of the web is written in HTML. Most of the browsers were designed for HTML. Postscript online requires add ons. Postscript add ons are quite often faulty. Clicking on a PDF file all but guarantees that my computer will crash and that I will lose all the work in open documents. This happens in both explorer and Firefox.
I've tested several other computers. About half of the computers I've tested crash when I click on PDF links. I've tested computers at local libraries. I know a person who bought a brand new computer. It crashed when I clicked on PDF links. I would not be surprised if 30% of the computers crash whenever someone clicks on a PDF.
Which brings me to my rant. I hate when people use navigation structures that mask the name and type of file in a link. I avoid PDF files like the plague. Invariably, however, so clever John will include PDFs with the name of the destination masked. I click on the link and before I know it, my computer is crashing as it tries to load the Adobe Acrobat Reader. I've lost hours of work because, while using the net to check data, I've acccidently clicked on PDF links. I hate Acrobat Reader with a passion.
Anyway, it seems to me that good web design etiquette requires that web masters make it clear when your link leads from an HTML page to a different file format. Personally, I think the primary navigation structure for a site should be entirely in HTML. The navigation bar at the top or on the side of the page should only go to HTML pages. The HTML pages should then make it clear when you are clicking to resources using add ons.
The same design criteria goes for contact pages. The main reason I click on a contact page is to see the physical address of a company. I consider it bad design to have a "Contact" link in the main navigation struction go to an email link. Anyway, I lost yet another pile of work to the fact that some bozo webmaster had a PDF link hidden in the navigation structure of a page. I clicked on what I thought would be an HTML page. Instead my computer does the Adobe Crash and Die.