Gets a value indicating whether the file is linearized: it starts with a linearization parameter dictionary whose file
length (L) is the length of the file.
public bool IsLinearized { get; }
ISO 32000-2 Annex F, Table F.1: a file whose length does not match "shall be treated as ordinary PDF file, ignoring
linearization information", which is what appending an update to a linearized file produces (G.7). Lengths count from the
%PDF- header.