Am I using async C# correctly?
You are using async/await absolutely correctly, that's how they were supposed to be used. 

Please note that from this code it's hard to tell whether the method will return the Task immediately, since it will be waiting synchronously for rasterize method to return the Task, and thus the answer depends on the implementation of rasterize.

As a side note - you are using implicit dependency here (first method prepares files in folder, and second method enumerates all pdf files there). I would prefer explicit dependency: rasterize to return a collection of FileInfo objects consumed by the concatenatePdfs method.