This is from Adobe InDesign (CS3) Help module:
Convert PageMaker documents
InDesign can convert document and template files from Adobe PageMaker 6.0 and later. When you open a PageMaker file, InDesign converts the original file information to native InDesign information. InDesign files are saved with an .indd filename extension.
For more information, see the PageMaker Conversion Guide (PDF) at www.adobe.com/go/learn_pmconv
Before opening the document in InDesign, you may want to do the following:
If the PageMaker file or its linked graphics are located on a network server, floppy disk, or removable drive, it may not open as expected if an interruption in data transfer occurs. To prevent data transfer problems, copy documents and their links to the hard disk, preferably in the same folder where the PageMaker publication is stored, before opening them in InDesign.
You may want to use Save As in PageMaker to clear unnecessary hidden data.
To ensure that all links are maintained, copy all linked files to the same folder where the PageMaker publication is stored.
Make sure that all necessary fonts are available in InDesign.
Repair broken graphics links in the PageMaker publication.
If you have a problem converting a large PageMaker document, convert portions of the PageMaker file separately to isolate the problem.
If you cannot open a corrupt PageMaker document in PageMaker, try opening it in InDesign. InDesign can recover most documents that PageMaker cannot open.