This file is invalid for use as the following: Security Certificate
This is not an error, it simply means you cannot open the file.
I've got the file with .txt extension and changed it to .cer, but that message appeared when I double click on it.
To view the certificates in the file, change the file extension from .cer to .p7b
Now, when you open the file and open the branch on the left of the window you will see 'Certificates'.
By clicking on 'Certificates' you will see all the certificates contained in the single .cer format file.
You can right click on every single .cer file and export it.
If you want to install it, you might use the .p7b file instead of the single .cer file as this could be a cross certificate.