I can help you with the following workaraound, that I accidentally found out today. It works with Windows 7 - 64 bit and Adobe Reader XI, maybe with other versions too. It´s only a workaround, but beware that maybe there will NEVER be a solution, because Adobe has removed that function intentionally - see here their comment from 2015-07-12, post #98:
Now here´s how you do it: Open Adobe Reader XI and browse for your PDF-files to open them (but do only browsing) via -> menu -> files -> open. In the explorer-like browser-window change the view to medium/large symbols - the icons are generated immediately, and are also displayable in windows-explorer later too!
Alternatively (the way I found it out), you can open a PDF-file in your browser (Firefox) by using the Adobe-plugin, then save the file, and save it again, but change the view to medium/large symbols, when you reenter the explorer-like browser-window.
Enjoy preview-thumbnails again on your Windows 7 machine!
Today (2016-02-23) I´ve installed Adobe Reader DC (latest version) and from now on my workaround mentioned above doesn´t work anymore! So I stepped back to Version 11.0.14 and above is working again.
Another convenient tipp to save time:
Launch Acrobat Reader XI and click on "File" - "Open" in the menu. Select the folder "Computer" on the left (be sure "Computer" shows up in the top-bar). Then type "*.pdf" in the search filter (at the top, right) and start the search (hit enter or loupe-icon). This way ALL pdf-files will get the desired preview-icons in one turn. Certainly you might have to wait some time till the process is finished.
[There are a lot of threads on superuser, all concerning this problem ("No Adobe Reader thumbnail icons in Windows Explorer on a Windows 7 machine"). Maybe it would be a good idea to put them all together, because I published my answer (facts from Adobe, workaraound, version information) only here.]