The character offset in the RawText to
the the Text of this fragment.
Each TextFragment spans a part of a PDF stream drawing operator.
The part may be the entire of the text drawn by the operator or it
may be a section of the text within that operator.
This property reflects the offset to the Text of the
TextFragment - the selected portion within the RawText of the
TextFragment. An offset may be needed because the Text may not be
unique within the RawText.
For example a TextFragment may represent the letter 'a' in a
text operator which draws "abracadabra". Without the offset there
is no way to know which 'a' the fragment refers to.