LectureNotes cannot directly handle PDF, it needs a free helper app PDFView (https://play.google.com/store/apps/details?id=com.acadoid.pdfview or http://www.amazon.com/Acadoid-Developer-PDFView/dp/B00CPR4L6K/) to convert the PDF file to its own format. Restart LectureNotes after installation, then there will be additional menu entries and the PDF import settings will be accessible.