Knowing that a .rdf file contains a list of resources that can represent triples (I understand that RDF files can be converted to a list of triples from this question: What's a RDF triple?, and also from having played around with the W3C RDF validator).
Can a collection of .RDF files represent all the data that is available at an RDF-compliant endpoint?
If you are asking about "Open World" vs. "Closed World" like in...
Statement: "Mary" "is a citizen of" "France"
Question: Is Paul a citizen of France?
"Closed world" (for example SQL) answer: No.
"Open world" answer: Unknown.
Source: https://en.wikipedia.org/wiki/Open-world_assumption
...you could follow this link: Open world assumption and SPARQL in triple stores