No. The filename extension is just a visual cue to help in identifying the document but there is actually metadata in the file (known as a file header) that fingerprints the file format. Operating system use abination of the two to determine the program used to open the file. So if you take a word doc and rename it from .DOCX to .PDF then you double click on the file the file will either open in word or try to open in your PDF viewer and then produce an error that the file cannot be read or some other related message. The behavior depends on you OS and the program used to launch the document to be viewed. A PDF file is actually a raw file that contains PostScript printermands. Over the years Adobe has added more data to these files but ultimately that is what this is. A file would qualify as a postscript document except that it is missing the PDF document metadata which is required by PDF readers.