Answer
As of now, there's no supported way to stream a File-Storage-repository-hosted document's content directly into a PL/SQL variable — this is a genuine architectural gap, not a missing method you just haven't found yet. Practical alternatives: use the database repository specifically for document classes where PL/SQL-side content access is a real requirement (impractical if you can't predict which documents need it, or if the volume is too high to want in the database); handle the file-content need through a Workflow instead, which may be able to call the right REST API to get a file stream back; or build something outside IFS Cloud entirely that reads the file via IFS's REST API, optionally triggered from inside IFS Cloud. There is an internal move-to-database-and-back pattern used narrowly in Financials/Invoicing, but it's considered an internal workaround, not a general-purpose technique to build new customizations around.