The file "..." couldn't be opened

Those are files that are not properly documented. They may lack Registered Developer Certificates, may not be properly notarized (if using the latest macOS, or may be 32-bit apps, or actually may be damaged.

Here is the official work-around for the first possibility:

Open a Mac app from an unidentified developer - Apple Support

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Dear Rebbie23,

I will try and help you, but first are you able to send a screen shot of the error message you are receiving and also what type of document you are trying to open. Eg. Xcl.x (Excel) or ppqx (Powerpoint)

Thank you!

Have you tried a file converter so that you can convert this document to a .pdf?

I have previously used pdfsimpli.com - press select a jpg to convert, find the folder, press 'lets get started' it should then say processing document, and then there should be an option on the top right corner to download. You can then choose which file format you want.

I really hope that this helps you, if not we can try something else! :)

Sometimes you don't have proper permissions for files.

Sometimes the files are damaged.

You are attempting to open with Preview, included free with MacOS. If the files are complex graphics files created with adobe apps, they may need Adobe Reader or Acrobat Reader DC to open them.