When you have such versions available:
apt-cache policy wkhtmltopdf
wkhtmltopdf:
Installed: (none)
Candidate: 0.9.9-4
Version table:
0.12.0-2~bpo70+1 0
100 http://http.debian.net/debian/ wheezy-backports/main amd64 Packages
0.9.9-4 0
500 http://http.debian.net/debian/ wheezy/main amd64 Packages
You can install version with lower priority with:
apt-get -t wheezy-backports install wkhtmltopdf