Check if every setting is in place. Go to the following place "C:\Program Files (x86)\Foxit Software\Foxit Reader\Shell Extensions" or "C:\Program Files (x86)\Foxit Software\Foxit PhantomPDF\Shell Extensions" to check if the required DLL and EXE shown below are installed.
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FoxitPrevHndlr.dll
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FoxitPrevhost.exe
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FoxitThumnailsHndir_x64.dll OR FoxitThumnailsHndir.dll (depends on your system type)
If there are files shown as above, then they are installed. Please exit Foxit program and any running File Explorer windows, do the following:
1. Right click on Windows Start 
, choose Command Prompt (Admin) to open Windows Command Prompt, type
regsvr32.exe "C:\Program Files (x86)\Foxit Software\Foxit PDF Editor\Shell Extensions\FoxitThumbnailHndlr_x64.dll"
OR
regsvr32.exe "C:\Program Files (x86)\Foxit Software\Foxit PDF Editor\Shell Extensions\FoxitThumbnailHndlr.dll"
2. Press Enter, if succeed, you will see the following message, click on Ok to finish it.
3. Open a File Explorer window see if the preview works this time.
If not, check the following:
1. In the Windows File Explorer, open the Folder Options, please uncheck option Always show icons, never thumbnails option, and check option Show preview handlers in preview pane .
2.In the Windows File Explorer, change the view layout to Medium or Large icons.
If the dll files are not listed in the installation directory, please try to re-install the software and see if it helps.
If the above items are ok and preview still fails, write a ticket to support by including the following information:
1) Descriptions together with screen captures to show how the preview not works.
2) What you have done in order to make it work.
3) What’s your operation system information, like Win 10 64 bit.
4) What program and version type of Foxit you are having.
