I have ACAD 2015 LT. Whenever I try to attach a pdf underlay autocad just stalls and never does anything. I can escape out of it so it doesn't lock it up. I have figured a way around it which is Opening a window outside of ACAD where the file is located and drag it into my drawing space in ACAD. That works fine but it is annoying that I can't use the actual pdf attach function.