Disable PDF page splitting by default.

[[:en:File:Wireless Networking in the Developing World (WNDW) Third Edition.pdf]]
is a 193M file which creates a 188M file for *each* of its 300+ pages when
given to pdfseparate.

Disable page splitting for PDFs by default to stabilize the OCG service,
until we implement a proper refactor to make the splitting demand-driven.

Change-Id: I9278e7dcac1e7d4fe2198136e53e5f338d4b35aa