Issue is caused by the following bug:
Bug 14248171 - DISCO PDF EXPORT : I/O ERROR WHILE STREAMING TO THE XDO PDF PROCESSOR
A fix is available for Discoverer 188.8.131.52, 184.108.40.206 and 220.127.116.11 and will also be included in 18.104.22.168 version.
Please create a SR with Oracle Support to receive the correct patch for your version as it depends on patches already applied on your install.
Note 1580199.1 - Export To PDF From Discoverer Plus 11g via a 64-bit Client Fails With Error: I/O error while streaming to the XDO PDF processor (Doc ID 1580199.1)
Note 1554490.1 - I/O Error While Streaming To XDO PDF Processor (Doc ID 1554490.1)