How to invert .p file to .m file

There is no documented way to convert p files to m files, and Mathworks will not assist you in converting files.

There have been at least three versions of p files. The first version was in use until about 2010. The second version was in use until a small number of releases ago, and the third version is now in use.

A small number of people outside of Mathworks demonstrated that they were able to convert version 2 p files to m files. None of the people who I was aware of ever provided a commercial service to do the conversion. I was aware of one person who provided a free service, but stopped doing so a number of years ago.

Whether it is legal for people outside Mathworks to convert third-party p files to m files depends on the jurisdiction. In the USA, the encoding done to produce p files would definitely qualify for protection in law, and so in the USA the only people legally permitted to decode third party files would be qualified security researchers, or university students in computer security courses acting under the direction of qualified security researchers — or, people with disabilities who were adapting programs for use with assistive devices for their own personal use (yes, this does mean that if you are someone who physically cannot do the adaptation then you cannot get anyone to do it for you! It is not a good law.)

The laws are different in some other countries, especially Germany, but the relevant laws are mostly driven by entertainment companies (copy protection / digital rights) who have managed to create fairly powerful lobbyists, so the laws are a fair bit more restrictive in many countries than most people would expect.