LINES appear when scrolling up a pdf

have Adobe Acrobat Reader DC and for the past few days when reading a pdf, which I open just by clicking on the pdf, when I scroll up heavy black horizontal lines appear on the portion of the pdf I have just scrolled up over -- and I have to close the pdf to make those lines go away. But then when I open the pdf again and scroll up, the same happens. I just changed operating systems to Mojave last month on my iMAC. Shortly after this happening my iMAC unexpectedly quit, the screen went black and I had to restart.