Trouble Editing Text

It has been several years since I have used Acrobat, please excuse me if I am asking something simple, but I can't figure this out. Now I have Acrobat X.
 
I have a document that I have downloaded and I want to edit it. It is a pretty simple document. Mostly text, some lines, and shaded boxes.
 
The crazy part for me: When I delete any part of the text the text above moves, or completely disappears. All I do is highlight and delete or I delete just using the backspace or delete buttons. Either way, the text above moves and/or disappears.
 
I am having headings that disappear. The lines that separate the different sections do not move. The text that was below the deleted text moves up and covers up the lines. In the end it looks like the remaining text has a line drawn through it.
 
This seems like it should be something simple, but I cannot figure it out.
 
Please give me some guidance. Besides being new at Acrobat, I just found out that I have to get this edited immediately!
 
I thank you ahead of time for your help, it is desperately needed.