A file's extension (the letters appearing after the period in a filename, such as "txt" in "details.txt") tells a computer what type of file it is and what type of program should be used to open that file. If a file has an incorrect extension, you may be unable to open it. It's relatively easy to change the extension on a PDF file on both a Mac and a PC, whether you need to change another extension to PDF or if you need to change a PDF extension to something else.
Open a Windows Explorer window. Go to "Tools" and "Folder Options." (If you can't see "Tools" in the menu, try hitting the "Alt" key.)
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Click the "View" tab and make sure "Hide extensions for known file types" is not checked. Click "OK."
Click once on the file you wish to change the extension for; this will highlight the file.
Right-click on the highlighted and file and choose "Rename" from the menu.
Override the current file extension (the section after the period) and change it to or from PDF depending on your needs. Click away from the file to activate the change.
Mac OS X
Right-click on your desktop. Go to "Finder," "Preferences" and "Advanced." Make sure "Show file extensions" is checked and click "OK."
Click twice on the filename of the file whose extension you wish to change. It should become editable.
Change the part of the filename after the period to or from PDF depending on your needs.