If it's a Pagemaker file in 6.5 or 7.0 format, saved in either Windows or Macintosh, you can choose File > Open in InDesign CS6 or earlier. This feature is not available for InDesign CC. However, CC users can download InDesign CS6 for free.
One thing to keep in mind is that the Pagemaker graphics should be linked NOT embedded or they may not be included. It's a good idea to open the file in Pagemaker first and make sure all the graphics and fonts are up-to-date. Pagemaker comes with a utility that globally unembeds and links graphics. Choose Utility > Plug-ins > Global Link Options.
Also keep in mind, that not everything will convert perfectly and you'll probably have to rework the file somewhat in InDesign.
Pagemaker 6.0 files will open as well. Other than that Steve's answer is