Several methods --
1) If you have the full version of Acrobat (not the reader) open the
document as save as RTF, which Word can open. This will retain the text and
images and at least some of the formatting.
2) If the document is reasonably short, you can copy and paste, from the PDF
to a new Word document.
Note that the PDF file's security can be set to block both of these methods,
in which case you may be out of luck.
3) Run the PDF file through an OCR program. (Results extremely variable)