Thank you for your answer, Francesco, but this is not what I am looking for. This would require me to add the document as an attachment to both all existing tasks, as well as every new task.

Ideally I would like to create a button that simply downloads the specified .xls file, and isn't related to a specific document.

That is unless I can automaticaly add my attachment to the project.task throught an inherit of the "Create task on SO" module or some other way? But not in a custom binary field, because this would mean I have open the form to get to my report.

Second me the only solution is aerro reports. Create an empty report, without placeholder and use it as a normal report. Insert a button in the form that call this report (as print button in sale order and account invoice)