From version 17.4 it is possible to configure the dynamic fields in Odoo in order to select freely which field we want to use for the PDF Quote Builder

From this menu, we have to fill three columns:

1st column: Document Type

It means if we will use a Header/Footer type PDF:

Or a PDF inside the product:

In this case (PDF inside a product), we have to set the visibility of the document as follows:

2nd column: Form Field

It means the name of the text box inside the PDF (free text)

3rd column: Path

The technical name of the field inside Odoo. 

Here, we can even use fields created with Studio or fields inside relational fields (like the client or the salesperson)

Examples of a new field created with studio, the name of the comnpany and the email of the sales person in the Header PDF:

Important note: 

The Header/Footer type documents will include the fields related to the sale.order model. 

The Product Document type document will be linked to the fields inside the sale.order.line model (the order lines inside the sale order)

Tip: Enable developer mode to see the technical names of the fields and their model

Example of field "name" inside sale.order.line

Update for V18

From V18, Odoo should automatically detect the names of the "Form Fields" (the fields inside the PDF) when the file is uploaded. Only the path should be added to the corresponding form field inside Odoo.

The button to configure the dynamic fields is now inside the document settings.