[Discuss-gnuradio] Converting from File to PDU

[Fred]

Have searched for an answer on the archives and either it is such a 
stupid question or I phrased it wrong, I could not find it.  Newb here.  
I am trying to create a transmitting chain that starts with a file and 
goes into an HDLC Framer for AX25 transmission through a GMSK 
Modulator.  I don't know how to convert the data in the file to PDU 
which is what the framer requires?  Sorry to bug you for what is 
probably obvious to most.
Best,
Fred