Unable to load or view pdf files in Finder

This issue just started a couple days ago. In Finder if I click on any pdf file I am not able to see the file in the small preview screen to the right of the file. If I try to select the Preview pgm in Finder I get an error telling me "The Application can't be found. I can't load a pdf file to an email. The file icon doesn't look red and white like a pdf icon.

What is going on?

Check your Applications folder and see if the "Preview.app" is listed there. If it's missing, it may have been moved or accidentally deleted. Do a Spotlight search for it.

It is in the Applications folder with all the other apps. If I open Preview in it's own Finder window or what looks like a Finder window with Preview at the top of the window, all the pdf files look correct with the pdf icon for a file icon and I can see the miniature file in the small window to the right of the file. If I use a regular Finder window I am seeing just a blank document in the preview window to the right of the file. I just clicked on Get Info on the folder where I keep all of my files, including all the pdf files and at the bottom of theInfo window is the Sharing and Permissions and there is a "Fetching" name there. I guess it can't find some files? I keep all of these files on an external drive.