Yes that did the trick for me:

Using Ubuntu 14.04:

1. First install the one from the ubuntu library. The executable goes to /usr/bin.
2. Then install the package from sourceforge with dpkg -i .deb filename. This will create (also) files in /usr/local/bin
3. then just simply copy the file over the one in /usr/bin

I had to reboot to get it working. Probably a restart of the openerp-server also does the trick.

I have manage to get it to work by doing the installation of wkhtmltopdf in ubuntu via apt-get install.

I was getting wrong version message so, I then copy the wkhtmltopdf file from the archive from wkhtmltopdf website and replaced the file that ubuntu installed in /usr/bin/.

It does work properly now. Dunno if I will get issues later on since I have not replaced the libs that comes in the archive.

Will keep updating this if any issue arise !

I made it work on Windows.

You have to follow this thread: https://www.odoo.com/fr_FR/forum/help-1/question/how-tu-print-a-pdf-with-qweb-wkhtmltopdf-v8-on-windows-tutorial-64326
The most important, do not confuse Environnement variables and System variables

We need do add the wkhtmltopdf path into Path System Variable and NOT in Environnement variables.
Do not forget to restart odoo server after this !

Thanks !

Anyone made it work on Windows? Where should I bind the binary...I put it in "PATH" variable... still no luck...

It gives me the same error and it loads in HTML.

I will eventually move to Linux... but Windows should work too right?

Thanks

THREE months past since this last question or request for an answer and no one replied from Odoo.... I am in the same trouble, installed Odoo v8 on Windows 8.1 Pro and cannot create any doc in pdf (nor in html) due to the same missing app (in spite of that I installed it manually. 

I think I should make it clear that the OP problem is that he is inserting the directory path of wkhtmltopdf to the webkit_path option. The correct value is the path to wkhtmltopdf executable. Below is the example of the setting, depends on your installation path.

linux: webkit_path = /usr/local/bin/wkhtmltopdf

windows: webkit_path = c:\program files\wkhtmltopdf\wkhtmltopdf.exe

Other option is not to use webkit_path. It can be done if you put wkhtmltopdf executable in environment PATH or the same directory as openerp-server script. I think the webkit_path option is more secure and easier to maintain, because it determine specifically what odoo can execute.

thank you Craig Barnes, it was the good way for me

Using Turnkey Linux Postresql - Debian Wheezy-  got Sales Orders and Invoices to Work with the following using SSH - no navigation from login, just run after login----This was after being unable to generate a PDF of SO or Invoice without errors previously------

apt-get install xfonts-base

wget http://sourceforge.net/projects/wkhtmltopdf/files/0.12.2.1/wkhtmltox-0.12.2.1_linux-wheezy-amd64.deb 

dpkg -i wkhtmltox-0.12.2.1_linux-wheezy-amd64.deb

apt-get -f install

(Then Started Openerp Server) Odoo