Hi @ismlylmz, and welcome to the forum.
. Rda is (older) file format used by R to save and load objects. Within an R-session you can load("my file. rda") . If the object is a data object, you can change values using regular R-assignments.
Excel files are either exported to '. csv' and then loaded into R through read.csv() or use the e.g. a package like readxl, which reads excel-files directly.
I don't know rdf-files and when to use the format.
Hope this helps, JW