Why does the inserted text on a PDF dim?

Using Acrobat Standard X my customer will edit a drawing saved in PDF format. While he is typing the text is quite bold. When he finishes with the information and goes to insert another piece of info the previously inserted line of text/numbers/symbols etc. dims. It is lighter than anything else on the drawing. What can he do to stop this? Previously he used 8 and this problem did not occur.