It's important to know that from Sign you can only access information from partners (res.partner). The only other possibility to have pre-filled fields is when generating an offer and to take fields from the contract.

That being said, to access the information of your customer, you can either take the fields already proposed (Name, email,...) or create new fields types. To do so, go in debug mode and create the new field type. By correctly filling the "Auto-Fill Partner Field" with the technical name of the partner field you will be able to retrieve the correct information.

For example, here is how to get the website field of a partner.

1. Create the Field type linked to "website", 

2. Drag and drop this new field in your pdf,

3. Ask the following contact to sign,

4. When signing, the partner will have his website field automatically filled in saving a lot of time,

Fyi, this also works with studio fields but not relational or related fields,