Clarification and assistance on File types and Extensions
The "Save As" dialog in Microsoft Word
In Windows nearly every file has an extension. The extension is the group of three or four letters after the final ‘.’ in the file name so for example EnableDKIM.ps1 has an extension of ‘ps1’ and will be opened by PowerShell if you double-click on it. The files that people mostly use are Word documents with extensions ‘doc’ for the older and ‘docx’ for the newer ones and, ‘pdf’ for the Adobe portable document format file you might send out to clients.
The file extension tells the system what program to use to open a file so if it’s wrong you will get an error or at least an unexpected outcome: we had a file with ‘pdf’ as its extension but it contained data in Word format so Adobe failed to open it and gave an error instead. It is thus very important to save a file with the correct extension. If you do “Save a copy” or “Save as” the file will stay as the type already expected and the extension should match that as you will see when you use the “Save As” dialog:
It expects to save as a Word (docx) but it can export to an Adobe (pdf) if required. If you don’t add the extension it will choose the right one for the type it’s exporting to – that’s the easiest way to handle it.
If you have two or more of you working on the same set of documents it’s vitally important to stay consistent so that each will be able to pick up where the other leaves off.
macOS uses a different system where there are two 'forks': 'Data' and 'Resource' the data fork contains the information and the resource fork, among other things, told which program created the file and opening it defaulted back to that. It is still possible to open the file using a different program of course.