How do I display / add the Terms and Conditions to the business document?

I would suggest that you create separate SMART Form for the Terms and conditions and call that right after the PO main smart form. It gives the flexibility in calling the same smart form for other documents also, as generally these are the same Terms and Conditions for all the documents that a company uses.

We can have the Terms and Conditions stored as long texts in SO10 (Standard Text, TDOBJECT - TEXT and TDID - ST and TEXTNAME - Whatever name you want to give). This way if you want to change the text, its easy to change and you can transport the changes using program RSTXTRAN.

We have a text element in the smart form, and the general attributes of the text change the type to "Include Text". This will change the screen and you should be able enter the parameter values of Object, ID and name.

This will automatically print the entire long text on the smart form, we just need to make sure that this is a part of the main window of the smart form as the text might run into multiple pages.