Currently there are several PDFs hosted on my site that I want to prevent from showing up in search results.
I have looked into several options such as using robots.txt, adding noindex through meta tags, or using a .htaccess file.
Since the PDFs already show up in the search results, if I use any of these methods, do I still need to manually remove them using Google's webmaster tools?
Would they disappear over time?
If yes, how long?
Since the PDFs already show up in the search results, if I use any of these methods, do I still need to manually remove them using Google's webmaster tools?
No, for future ones from that point onwards provided those rules you set are respected by search engines' crawlers, and Yes, to speed up the removal process of the old ones.
Would they disappear over time?
Removing them using Google's Webmaster Tools, yes; they will be removed after some time.
If yes, how long?
I can't really tell; it may vary depending on prevailing factors and/or conditions surrounding Google interpretation on your intended purpose.