.PM$ File

The ​pm$ file extensions is related to Pegasus Mail and used for its temporary files that are created, when parsing attachments and/or html files.

Those files should be deleted by Pegasus Mail, but are not. You can delete them yourself, but make sure Pegasus Mail is not active.

How to open:

In vast majority of cases, users cannot access temporary saved data.

How to convert:

Temporary files are usually not meant to be converted to anything else.