How can I restore Acrobat icon thumbnails?

My assumption is that at some point the Garbled folders in your C:\Windows\Installer directory were cleaned up (aka deleted). I know of people that have actually thought these are temp folders for the installer and deleted them...

If thats not your situation, this fix should help you out anyway by changing the icon type for the pdf file using the Windows Registry Editor back to the right one.

WARNING: If you use Registry Editor incorrectly, you may cause serious
problems that may require you to reinstall your operating system. I cannot
guarantee that you can solve problems that result from using Registry Editor
incorrectly. Use Registry Editor at your own risk.

I've located the directory where PDFFile_8.dll is by pressing F3 when in 'windows explorer' while in my windows directory. For me the path is:
C:\Windows\Installer\{AC76BA86-7AD7-1033-7B44-A90000000001}\PDFFile_8.ico
Remember this, you'll need it in a sec...

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---- Find the key you'll have to manipulate
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Here is the full instructions on how I located the information, it doesn't actually mean you have to do it. Its just a good lesson. Scroll down to a recap on the key you'll need to add.

1. Click on Start, and in the Start Search field type "regedit.exe" and then
press Enter.
2. If prompted by User Account Control, provide consent or provide the
appropriate credentials.

Next, you will need to locate the file extension you wish to modify. For
instance, if you wish to change the file type icon for .PDF files, locate
HKEY_CLASSES_ROOT\.pdf in the left hand navigation pane. Now, you will see
that it is defined as type "AcroExch.Document", so we must locate "AcroExch.Document" in the
left hand pane to change the icon.

We must keep tracking it down until we locate the DefaultIcon key so under the AcroExch.Document string you will notice that it points you to AcroExch.Document.7

Now we find AcroExch.Document.7 on the left hand side (it should be right below it)

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---- Now change the value that we have located
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At this point you should have located HKEY_CLASSES_ROOT\AcroExch.Document.7. Below this
key you will notice a subkey with a name of "DefaultIcon". Select this key,
and in the right hand pane you will see a string or expandable string value
named "(Default)". Right click on this string value and click on modify in
order to change the path to the icon.
Because you are trying to change it to the default icon we'll insert the icon you located above and add a ",0" to the end of it to tell the system that its the first icon in the file. Mine looked like this:
"C:\Windows\Installer\{AC76BA86-7AD7-1033-7B44-A90000000001}\PDFFile_8.ico,0"

Once you have finished, close the Windows Registry Editor and log off of
Windows and back in for the changes to take effect.

I hope this works for you... Its likely that this isn't the cleanest method to getting it done, but it should work for you. Best of luck...