PDF: Portable Document Format (*.pdf) exports the active document as PDF-file. It is also possible to use the normal Save As... function to save in these formats, but the Export-functions have the advantage, that the name and the changed-state of the document will not be modified. This way the risk of forgetting to save in the PLD-format - the only file format that can save without losing any information - is minimized.
Of course, you can change the file format in the following file panel, and export in a different format.