Get data from pipe

I'm writing an interface in RPG to a C api using the pipes
to get the output from the api.

This api - depending on the parameters passed - can return
a list of documents that are on the optical disk or the
document itself (that can be a pdf, a jpeg image or any
other file).

In the first case I can retrieve the items in the list one
at the time from the spawned job pipe with "fdopen" and
"fgets".

How can I get the full document instead ?

Thanks.