Proteus Design Suite allows users to export its own projects (.pdsprj) to electronic design files (.dns). This pretty much covers pdsprj to dns conversion, but is more a standard workflow in the program and not actually file format conversion in traditional sense.
Convert Proteus project to Proteus Design Suite electronics design.
We found one software available in the database for this conversion.
Microsoft Windows software - convert pdsprj to dsn on Windows
A PCB designing tool
Convert from pdsprj file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.