How to convert .rda database to excel workbook
Just because a file is names ".RDA" does not guarantee it is Oracle.
It could be Oracle, but then it would/should come with a whole infrastructure, not just a datafile.
And if it is Oracle, then you'll need Oracle to get the data out.
You may be able to use the EXPRESS EDITION depending on how official this is.

You need to speak to the 'owner' of the RDA file.
Where did it come from, how did you get it, what is  supposed to be in it?

Personally I would just use an Editor with HEX extension (Notepadd++, Textpad,...) to eyeball the contents.

Hein