Do View and Update specify pdflatex? Am I understanding correctly that File > Export PDF (pdflatex) produces a correctly compiled PDF file?
If the answer to both those is 'yes', then perhaps LyX does not know how to display a PDF file. Look at Tools > Preferences > File Handling > File Formats. Select PDF (pdflatex) as the file format, and see what LyX shows for the Viewer entry (Custom - evince on my Mint system, but yours may well be different). Paul
