System.FormatException: Too many bytes in what should have been a 7 bit encoded Int32.
The issue was related to a divide-by-zero 'error' in one row in the result set from an Analysis Services cube. Analysis Services doesn't actually error in this situation but returns '1.#INF'. The HTML version of the report happily rendered 'Infinity' however the PDF renderer would appear to be using the BinaryReader.Read7BitEncodedInt Method of System.IO and this obviously chokes on the string since it expects a 7-bit integer. Hence the slightly obtuse error message in the Reporting Services log above. Removing '1.#INF' is as easy as doing the following:
WITH MEASURE c AS (IIF(Measures.a=0, NULL, Measures.b/Measures.a))
Mosha commented here that he didn't see the point in removing these values. I guess he hadn't had to render PDF's using Reporting Services at the time!
After attempting to reproduce the error it appears that Reporting Services normally, is quite happy to render PDF's correctly, even when there are '1.#INF'-type characters in the results. This is the case even when switching from an integer-only field. So the bug is obviously much subtler than it first appeared. I'll post about this again if I figure it out...