Have you tried without modified extensions.cfg?
pdfviewer was a standard app and selectable via "open with" function from library.
Is your line in extensions.cfg correct? should look like this:
"pdf:@PDF_file:1: pdfviewer.app,AdobeViewer.app:ICON_PDF" (please remove the blank between : and pdfviewer... , but it shows a smiley without the blank