Thank you Slava!
It now works!
So, in case some other users struggle with same issue, here is the fix for CentOS:
1. Download and unzip the static binary of
wkhtmltopdf and extract the archive under "
/usr/local/sbin".
2. yum install dejavu*
3. Go to BILLmanager -> Global Settings -> Settings (TAB) -> "Path to the HTML2PDF converter file" -> /usr/local/sbin/wkhtmltopdf-i386