Select a range of text in the document. If GetText has not been
called previously then an exception will be raised.
This function is purposely designed to be similar to the
String.Substring function. The concept is that you can use the text
provided via the GetText method to identify pieces of text you are
interested in. Then using the offset and length of the pieces you
can select the actual text in the document.
The selection specifies the precise set of text fragments in the
document. Each fragment has an area, some text and relates back to
a text drawing operation within the content stream of the page.
Applying the Group function to these
fragments can group connected fragments into connected parts.