not sure if this was a configuration error on my end?
while setting up a new 1.7.8 server all plug-ins were detected automatically except wkhtmltopdf (installed wkhtmltopdf using apt in ubuntu).
had to change the path in external_applications.conf from
wkhtmltopdf_app = /usr/local/bin/wkhtmltopdf
wkhtmltopdf_app = /usr/bin/wkhtmltopdf
also not sure if this note on the 'installling providence' wiki page is still the case with wkhtmlpdf 0.12.4-1 released? "Version 0.12.1 is supported. Do not use version 0.12.2, which has bugs that impede valid formatting of output."