Sorry but .DAT files are not defined except by the application that uses them. There might be a way if you can reveal what APPLICATION you use with this .DAT file.
A .DAT files is a file that has arbitrary data. That means it's not associated with any one particular program or application.
If I had to attempt this, I would open the .DAT file using Notepad, (in Windows), then re-save it as a .TXT file.
Then Google How to convert a TXT file to PDF. For example;
Hoarding photos on your phone?
Those picture are hogging memory and could be slowing down your phone.