Adobe ACROBAT READER 7.0 Manual page 176

Selecting a reading order
On pages with multiple columns or stories, the natural visual progression through blocks
of text may be complicated, especially if the page design is complex or if the document is
poorly structured. Choosing a reading order can improve how untagged Adobe PDF
documents are read, by both screen readers and the Read Out Loud feature. The reading
order also affects the order of text when you choose File > Save As and select the Text
(Accessible) *.txt option.
Adobe Reader includes the following reading-order options:
Infer Reading Order From Document (recommended) delivers words according to a
sophisticated structure-inference process that determines the most likely reading order.
Unless you experience unsatisfactory results with a specific document or have
performance problems when using this option, leave this option selected.
Left-To-Right, Top-To-Bottom reading order reads the text according to its placement on
the page and is faster than Infer Reading Order From Document. This option analyzes text
only; form fields are ignored and tables aren't recognized as such. You may use Document
mode with this setting, or type a large number for the Minimum Number Of Pages In A
Large Document setting.
Reading Order In Raw Print Stream delivers words in the order in which they were
recorded in the print stream and reads documents faster than Infer Reading Order From
Document. This option analyzes text only; form fields are ignored and tables aren't
recognized as such. You may use Document mode with this setting, or type a large
number for the Minimum Number Of Pages In A Large Document setting.
Override The Reading Order In Tagged Documents. Select this option only if you
encounter problems reading a tagged document.