To replicate:

1) Install confluence as a war file on Resin app server (tested on 3.0.24 and 3.1.4a)
2) Add attachment with two consecutive spaces in the name
3) Export to pdf

Log error (see attachment)

@40000000476070c206613b6c java.lang.NullPointerException
@40000000476070c20661433c at com.caucho.server.connection.IncludeResponseStream.flushChar(IncludeResponseStream.java:292)
...

The problem appears to be limited to Resin, or at least it doesn't happen on Tomcat.

The workaround is to rename the attachment without consecutive spaces.