Adobe Reader will provide both thumbnails (1st page of the document) and viewers/previewers, and I guess is what most people use.
If you are on 64-bit Windows, Adobe still haven't fixed Reader to generate thumbnails, but I fixed it for them:
http://pretentiousna...f_x64_fix/index.htmlWith Adobe Reader, plus that fix installed (or on 32-bit Windows), you could get an instant preview of the first page of all PDFs by switching to thumbnails mode. In Directory Opus you can also get the same thumbnails in the infotip you get when hovering over a PDF, if you put {thumbnail} into the PDF intofip definition. (Or, like I do, into the All Files infotip definition.)
You can also open the preview pane for Adobe's preview handler, which lets you scroll through the whole file. (The same preview handler that Windows Explorer uses, although in Opus we have some extra code that detects Adobe's preview handler and fixes several bugs in it, e.g. not repainting properly and not responding to the Tab key correctly. There's a list of those on the page I linked above.)
Last I looked, which is a while ago and may no longer be true, most other file managers didn't support preview handlers but would show PDFs via Internet Explorer which in turn would use Adobe Reader's ActiveX control. You can use that in Opus as well if you want, but it's not the default as it doesn't work very well on Windows 7 and/or later versions of IE, thanks to Adobe's brilliant programming no doubt.
PDF-XChange is an alternative PDF viewer that supports both thumbnails and preview handlers, without needing any fixes like my ones for Adobe, so you might want to look at that as well, and how well it works with Windows Explorer or whichever file manager you prefer.
