Adobe PageMaker is a discontinued program and its documents (*.pmd) are not well supported in other software, so your only option would be to get your hands on PageMaker and use the default export as text function to save your *.pmd files as plain text files (*.txt).
Convert Adobe PageMaker document to Simple text file.
We have found 2 software records in our database eligible for .pmd to .txt file format conversion.
Microsoft Windows software - convert pmd to txt on Windows
One of the first desktop publishing programs on market
Convert from pmd file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned pmd to txt. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert pmd to txt on OS X
Mac version of once popular publishing program