Problem:
When viewing PDF files in Windows Explorer, thumbnail images are not displayed.

Solution:
The thumbnail images should automatically be generated by the default PDF viewer which is normally Adobe Acrobat Reader. If they aren't being displayed, try the following:

1. Install the latest version of Win2PDF Desktop from:
   https://www.win2pdf.com/download/download-win2pdf-ocr.html
   New versions contain a fix for file associations that may be interfering with the normal thumbnail creation process.
2. Make sure you have the latest version of Adobe Reader installed, and that it's set as the default application for PDF files:
   https://helpx.adobe.com/acrobat/kb/not-default-pdf-owner-windows10.html
3. Run the Windows "Disk Cleanup" utility, and only check "Thumbnails". This will force Windows Explorer to rebuild thumbnails for all files.