While downloading some games I came across some files in Chessbase format:
I would like to know if is is possible to convert these files to .pgn format without using proprietary software and how it can be made.
Well since the protocols creating cbv, cbf and cbh formats are not made publicly available, one will certainly not find any such conversion tool (i.e. unclear how they are encoded to reverse them to pgn).
Other reason maybe that the difference between all these formats is just due to an "incremental encoding" that is being used, which means that for it to be possible to go backward, to a pgn format, only the chessbase engine can do so!
choose file->new database and create pgn files
will allow you to obtain pgn formats again.
Here is a simple way to do exactly what you want:
You now have a .pgn database!
There is now a CBH to PGN app for Android (only handles CBH files, not cbv) - https://play.google.com/store/apps/details?id=org.chess.cb&hl=en
I personally would like to see this available as a command line utility on windows.