(Part of the reason is that PDF has been around long enough that better tools have evolved to replace the astoundingly awful Adobe Acrobat of yore. Thank you, Apple Preview! Thank you, Foxit Reader! Thank you... um, whoever wrote that Linux PDF reader that I can't remember the name of, but which comes with Ubuntu and just works!)
// in fact, it's almost exactly like that, since the latter (Scribd/YouTube) is just a presentation layer for the former (PDF/WMV)
EDIT: Could the person who downmodded me please explain? I'll delete my comment afterward. Thank you.
EDIT2: After reading the two posts below, I now redact everything in this comment. :) I didn't say what I meant. I agree, I have no problems with PDFs either. However, I don't have a problem with WMVs either, since they open flawlessly in VLC. My point was that it's funny because I've gotten used to the worldview of "Oh noooo, PDF!" (and similarly, "oh noooo, WMV!"), and now it's kind of funny to see the opposite happening ("here's a PDF!" instead of screams of anguish).
Also, Paul, this is why I add a comment edit requesting clarification each time I get downmodded to 0 without a reply (and suggest my peers do likewise). Not because I disagree or find it questionable (I don't), I would simply like more feedback than that and hope it instills that habit into people reading my edit and the person who modded me down, since feedback is what makes Hacker News.
PDFs don't bother me at all. Years ago I would occasionally accidentally click on one and be filled with dread and loathing as the Adobe Reader plugin slugged to life, but that was a long time ago. I use Safari's PDF reader now, which does what and (more importantly) behaves like I expect it to.
Scribd documents don't behave like I expect them to. Pressing down arrow scrolls further than I expect it to, mousescroll is cadenced differently (and jumps around), and I can't press space to page down.
Scribd is definitely a solution to some problems, but—for me—reading PDFs is not one of them.
I upmodded you to balance it out, but it seems lately that some people are using downvoting to mean "I disagree", regardless of the quality of the comment. It seems that most sites with up/downvoting eventually turn into popularity contests for opinions, but I've always hoped news.yc could do better than that.
That said, I do disagree with your comment :). While I do like the Scribd reader, the problem of viewing PDF documents has been solved by a number of third party viewers which do not suck as much as Adobe's (as others have mentioned).
In my opinion the difference is that PDF is the lesser of two evils when it comes to PDF vs. Flash, whereas Flash is the lesser of two evils when it comes to Flash vs. WMV.
You haven't considered the difference in the formats. A key advantage of YouTube over WMV is that it loads instantly, makes streaming-while-it-downloads simple and transcodes into a quick-to-load low-bitrate version, which just completely doesn't apply to PDF - viewing PDFs using Scribd actually makes things slower and more clumsy /not/ faster or more accessible from my end. PDF is also a open format, a de facto standard and cross-platform compared to multiple codecs for video which can make life harder than necessary for viewers. Another key difference is that Scribd is used by the site as an unwanted front-end to content readily available elsewhere - compare this to YouTube which is often the exclusive host of their content and so there isn't a choice.
FWIW, when an alternate format is available for a YouTube link I often use them too as it makes life easier for managing windows or deciding to keep on the desktop knowing I won't need a net connection or have to buffer later. A recent example was John Resig's talk on the future of jQuery which was available as a download on Vimeo.