WkHTMLtoPDF is a tool to convert html reports to PDF. Odoo/OpenERP uses this tool to generate PDF reports. If you are using an unicode character in PDF report, that character has to be installed in font package in the system. If you are using Windows, Bangla characters are available by default. But that is not same for Linux.
Bangla & Assamese fonts are available for Linux, in a package called ‘lohit’.
To install Bangla fonts in CentOS/Redhat based system, use the following:
yum install lohit-bengali-fonts
To install in Debian/Ubuntu based system, use the following:
apt install fonts-lohit-beng-bengali
Once this is installed, PDF reports should start showing the Bangla characters properly.