Some file formats such as .pub, or .wdb – may not open on a MAC as they need the file reader or program that created the file.
The .docx file can be opened on the Mac if the Microsoft Office Word program has been downloaded.
Mac file formats like .pages can not be read on a PC unless saved as a PDF or word file.
There are many different types of file formats – if you cant read or open a file – do some research to find out why.
In your search engine ask the question
E.g.: can’t read .pub on Mac (you will find that the sender will need to convert the .pub to an interchangeable format such as .PDF or .JPG