Why don't you just copy the RPD and use it as reference?
Sure you can generate the RPD documentation and get a file with bunch of relationships inside, but what better than the actual RPD as reference? Or even better just start doing your changed directly into the copy of the RPD, so no need recreate everything.
Thanks for your reply.
So, it is possible to open the copied RPD file with any Admin Tool even if we don't have the same DB connection. Is it right?
Once you get the RPD file it's really just a binary file with no dependencies or anything.
You can open it with "many" Admin Tool (not "any": the Admin Tool need to be ideally same version or higher, a older version can't open a more recent version RPD).
The DB connections inside are defined, but don't need to work to allow you to open your RPD, of course if you want to use it for real you will need to update them to get some data being retrieved.