I got this problem here too, after while searching for a solution... I figured out the problem was with wkhtmltopdf, you have to get install the version 0.12.0 or above, and test if it's  working alone. the current version is 0.12.1.

For me this problem happens because wkhtmltopdf was generating pdf with zero bytes, throwing the error not handled by the code.

To get wkhtmltopdf working, the easy way is to download the lastest version from http://wkhtmltopdf.org/downloads.html

To test if working you can try at command line, preferable no xorg running:

wkhtmltopdf http://www.odoo.com test-odoo.pdf

If xorg running try this:

DISPLAY=  wkhtmltopdf http://www.odoo.com test-odoo.pdf

if you got error "wkhtmltopdf: cannot connect to X server", you have a wrong version of wkhtmltopdf.

After that, if you can create the pdf file from command line you can try to send the email with attachment(if so) again.

Check if pdf file created is ok, go ahead and to test again send email.

I hope help you, comments my answer if you need assistance.