As macpawel suggested it is probably a case of having what is called "Overset Text" somewhere on the first page of your document. This means that there are one or more text frames on that page that have more text in them than can be displayed at the frame(s)' current dimensions. To find out which frames look at the lower right corner of each text frame on the page and see if there are one or more frames with a square with a red cross inside as shown in the screen shot. If so, open up the frame(s) until all text is shown and the red square turns into an empty square that is outlined in the same color as the rest of the text frame (the color of the frame is determined by what layer it is on). Once there are no longer any red crosses showing then try making a new pdf.

I will also reiterate the question that Frans asked as to why you are opening the pdf in Illustrator instead of Acrobat?