Generally, you can copy and paste the text from any document format, including Microsoft Publisher PUB and save it in plain text format. Any special features, graphics and formatting will be lost however, and you will only keep the text.
Found 2 different conversion types using the same filename extensions.
Conversion pub to txt type 1:
Convert from Microsoft Publisher document to Simple text file.
Multiplatform software - convert pub to txt on any platform
Word processor and text editor from OpenOffice.org suite
File type (from-> to)
Convert from pub file format
Microsoft Publisher document
Simple text file
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 pub 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.
Conversion pub to txt type 2:
Convert from Publication document file to Simple text file.
Microsoft Windows software - convert pub to txt on Windows
One of the first desktop publishing programs on market
Apple macOS / Mac OS X software - convert pub to txt on OS X
Mac version of once popular publishing program