This is an Oracle question. Is it a BLOB or a BFILE? Normally, Oracle stores BLOBs inline (in the column, in the row of the datafile). When you store the .pdf on the filesystem, the table column only stores the LOB locator, not the actual binary data. To access the filesystem for a BFILE, you build a LOB locator by putting together the directory name plus the filename. (the directory part is the alias from the Oracle "create or replace directory" command)

Your error may be that you are reading a LOB locator and thinking it is a .pdf ??

Please specify more, and maybe I can answer more specifically.