Again, we need more information. Are you running a Windows, Mac, or using a phone or tablet? Were you sent the file in an email? What is your email program? How did you open the pdf file? Did you download it to your computer or did you just click on a link in the email. If it is the later, then the most likely thing that happened is that the pdf file was loaded into a temporary file area of your computer for viewing in Reader. Under Windows the temporary file, will eventually be deleted by itself, on the Mac it will be stored in a special area of your user folder and you can go in to it and manually delete the file.
We cannot be more specific without more information on your part.