Adobe ACROBAT READER 7.0 Manual page 168

Understanding Reflow
In Adobe Reader, you can reflow a PDF document to read it on handheld devices, smaller
displays, or standard monitors at large magnifications, without having to scroll
horizontally to read each line. Reflow facilitates only the reading of documents; reflowed
documents can't be printed or saved.
Adobe Reader assigns a temporary tag structure to untagged PDF documents when you
select Reflow to improve the reading order of the document. (Tagged PDF documents
already include a tag structure that helps the reading order of reflowed documents.) All
readable text reflows into the reflowed document in a logical sequential order. Readable
text includes articles, paragraphs, tables, images, and formatted lists. Text that doesn't
reflow includes forms, comments, digital signature fields, and page artifacts, such as page
numbers, headers, and footers. Pages that contain both readable text and form or digital
signature fields don't reflow. Vertical text reflows horizontally. (See Reflowing the
contents of Adobe PDF documents.)