If you save the Max/MSP patch as a "Max patch file" in the pat/mxt text format, then Pd can open that file. Pd will not open a pat file in the binary format. The tricky part is that all of the objects have to have the same names and be compatible in order for the patch to work.

You can also save files at "Max patch files". On GNU/Linux and Windows, you can select the file type at the bottom of the Save panel. On Mac OS X, just change the .pd at the end of the file name to .pat, and Pd will save using the "Max patch file" text format.

Krzysztof Czaja's Cyclone project is aimed specifically at Max compatibility and includes a loader for binary and text Max patches. It includes a cyclone object which is meant to ease the conversion. To use it, load cyclone as a library with "-lib cyclone", then create an object "cyclone", click on it and use the File-Open-dialog to open a Max patch.

Max 5 introduced a new JSON-based file format with the extension .maxpat. Max 5 can still read the older Max 4 format .pat and .mxt formats that Pd can export, but MaxMSP 5 can no longer write .pat or .mxt format files that Pd can directly read. Cyclone also can not read the new .maxpat file format.

Fredrick Olofsson has written a SuperCollider patch that can convert a Max 5 .maxpat file into a Max 4-compatible .pat file. The .pat files resulting from the conversion can then be opened in Pd or Cyclone. Fredrick Olofsson's MaxPat converter