I am a beginner and user, not web developer. I have downloaded Redlettuce's PDF viewer for Elementor and can upload and display a PDF file in 'build' mode. However, when I go to 'preview' all I get is a black, blank area where my PDF should be! How do I embed the PDF document into the website so that the document displays in public? Should I be using the PDF widget or the PDFJs widget?