Most often the text runs you would extract from a PDF are in the order you would read the text. Probably because the word processor or application that created the PDF dumped the text into the capturing PDF context from its own text container that, in fact, contains the text in the order the word processor would display it (editing, searching, text selection in the creating app obviously benefit if the text container is in reading order).
When text is not in reading order within a PDF page it is often headers, footers, captions, callouts, block quotes....
There are I believe features in the modern PDF spec to allow for accessibility that would give you more structure to that raw text. I am not sure that this is a widely used feature when creating PDFs though.
