Outlook- This file cannot be previewed because there is no previewer installed for it

Solution 1:

I found a post mentioning that installing Adobe Acrobat Reader would enable PDF previews in outlook, so that's one solution, but that program has always been far too slow at opening PDF files for me to use it as the default viewer in Windows. I had originally set the portable version of SumatraPDF as my default PDF viewer because it is super fast and light, but previews still weren't working.

I had to get the full 64bit installer version of SumatraPDF, open it and click "Options", enable the checkbox for "Let Windows show previews of PDF documents", and then set SumatraPDF to my default viewer to get previews to work in Outlook.

I think you also have to match the bitness (32bit vs 64bit) of your Outlook app and the SumatraPDF installer (or whatever your preferred PDF viewer is). To check bitness in Office 365 Outlook, go to File > Office Account > About Outlook. (On the top right mine says "64-bit" at the end of the version number).

To select a preferred program to open PDF files by default (the typical/easy way), click on the Windows 10 start menu, start typing "Default App" and open "Default App Settings" (or go to Windows Settings > Apps > Default Apps), then scroll down and click "Choose default apps by file type". Scroll down to ".pdf", click on the current app shown to the right of it, and select your preferred app/program. (In Windows 7 it's Control Panel > Default Programs > Associate a file type or protocol with a program).

(If it doesn't show your preferred app in that selection popup (and you've already installed it), or you want to make sure you're pointing to the correct .exe (like if you're switching the default program from the portable version of a program to the normally installed version), try this next method):

To select a preferred program to open PDF files by default (the more accurate way): Right click on a pdf file, click "Open With" > "Choose Another App", enable the checkbox for "Always use this app to open .pdf files", scroll down and click "More Apps", scroll down again and click "Look for another app on this PC", go to C:\Program Files\SumatraPDF and double click on the SumatraPDF.exe file in that folder (for example). You may need to close and reopen Outlook.

If previews still don't show up (and you've restarted your computer, and updated Outlook [through File > Office Account > Update Options > Update Now]) this link mentions another possible solution:

I first installed the 32 bit version. Copied the installed files to a backup location. Then uninstalled the 32 bit version. Then installed the 64 bit version. Copied back the 32 bit version to the "Program Files (X86)" folder. Started a command prompt. Then I typed: regsvr32 C:\Program Files (x86)\SumatraPDF\pdfpreview.dll Now it seems to work in both Windows Explorer (64bit) and in Outlook 2013 (32 bit)

If you want Foxit Reader to be your default PDF handler, according to a comment on this question you can try installing the "Foxit PDF Preview Handler" (I haven't verified these files, use at your own risk): from here (direct link 1, direct link 2, XP direct link 1, XP direct link 2).

This info is for Office 365 Microsoft Outlook, desktop version 1906, on Windows 10 (2019-06-28).