Odoo is the world's easiest all-in-one management software. It includes hundreds of business apps:
CRM | e-Commerce | Accounting | Inventory | PoS | Project management | MRP | etc.
Printing a single report (sale order, delivery slip) works fine but if I select a few in a tree view I cannot print at once.
The resulting PDF is blank. Is Odoo able to do that out of the box?
Yes, Odoo is able to print multiple reports at once after selecting them from the tree view out of the box. I've just double checked it and it works fine on the official Odoo instances (at runbot.odoo.com).
Are you sure that you're running the right Wkhtmltopdf version, 0.12.5, as suggested in the wiki at https://github.com/odoo/odoo/wiki/Wkhtmltopdf#version-status ? Did you update your Odoo to the latest source code?