As Laura said, phones don't open files, apps do. You need an app that opens the particular type of file you're trying to open. Register an account here, so you can post in this conversation (guests can't post to existing conversations, and we don't want to go through all this again when you post "a pdf file", because we won't know what you're talking about). Then come back to this conversation and add a post, telling us what type of file it is, and someone will give you the names of some apps that can open it (or them if there's more than one type). Pdf files are one type that Android phones don't come with an app to open, but there are others (there are over 600 file types that I know of, and I'm far from knowing them all).