How to allow my customers to do online signature on my quotations PDF that I sent to them? or how to include an ACCEPT button/link in the emails for each quotation I sent to them?

Would love to know if something like this is possible

Buongiorno, non penso che sia possibile ad oggi questa sua richiesta. 

Nel mio caso ho creato un modulo google, che invio al cliente insieme al preventivo. Sulla mail di arrivo al cliente ho impostato un link, dove se il cliente accetta il preventivo, clicca sul link, si apre il modulo, lo compila e lo invia al mio ufficio.

Have you looked into DocuSign?

http://bit.ly/2QCis3n

Well sending an email from inside Ninox in HTML format is possible, so with an HTML button surely you could do an API call back to Ninox updating the record of acceptance?

Just a wild guess.

Would Integromat or Zapier provide any options?

I like Antonello approach.. and I do not even speak Italian.. LOL..   

Depending on the volume... I would combine that Google Form approach with Integromat that would auto populate your Ninox database. 

If it is to be "legally binding" .. then @Scott's suggestion will work.. doing a GTS (google that stuff) I see a Zapier integration.. and DocuSign has a REST API which suggests you can use Integromat with Ninox.. 

I would not do the embedded button within an html email unless you have some way to protect your Ninox API key..