First things first...
Be sure you are opening the file in Adobe Reader. If you use any other program, it may not be compatible with the editable features. (This is also true at times with non-editable PDFs, so anytime you have trouble with a file looking funny, try opening it directly in Adobe Reader or if you are already in Reader, check to see if you need to update your version.) You can download Adobe Reader free here: http://get.adobe.com/reader/
On a Mac, the default program to open PDF files is called Preview. This is NOT Adobe Reader, and it frequently results in frustrated buyers who have files that look all wonky! You will need to right-click and manually select to open the file with Adobe Reader. (This is the number one problem my buyers have had with PDF files!) For more information on how to open in Adobe Reader, check out this tutorial.
Now that you're in Adobe Reader...
You should see greyish-blue text fields wherever the creator has put editable text.
Click in any of those text fields. Then right-click in a blank area of the toolbar at the top of the screen. Select Properties Bar from the drop down menu.
Here's a quick look at all of your options. You can change the text color, orientation (right, left, center), font and size, font properties (bold/italics/underline), and add links. Fonts that were embedded in the file by the creator will still work on your computer, even if you do not have them installed.
The More... button at the end of the toolbar will allow you to further edit the file.
In the FONT tab, you can choose your font, size, color, and properties. I have found that when font changes don't "stick" after I change them in the Properties Bar, this usually works better.
In the PARAGRAPH tab, you can align the text vertically and horizontally and adjust line spacing.
In the LINKS tab, you can add hyperlinks to the selected text.
FYI - This tutorial was created using Adobe Reader XI. The editable product pictured is my Substitute Need-To-Know Flipbook, available in my TPT store.