Sometimes when you increase the zoom level on a document the text will run off the side of the screen. In order to read each line you have to scroll left and right which can make it difficult to read the text. Reflow converts the document into one column so that you don't need to scroll left and right to read it.

1. Open Adobe Reader DC
2. Click on the Edit menu, then Preferences
3. Under Categories – click on Accessibility
4. Put a tick next to Always use Zoom Setting and change the drop down box to Reflow
5. Click OK
6. You now need to close Adobe Reader XI and open a PDF file for the changes to take effect

Some documents may not display correctly with reflow on so to temporarily turn it off:

1. Open the View menu
2. Go to Zoom then click on Reflow to turn it off. You can turn it back on in the same way 

Unfortuantely this may not work with all PDF files as it depends on how the PDF was orginally created.  You may be able to use SensusAccess to convert the PDF into a tagged PDF which may allow the text to be reflowred.  Alternative you can use SensusAccess to convert the PDF into a Microsoft Word document which will then allow you to change the text size.