Footer on all Pages

We need to add a footer option (just like the header) to add our company details at the bottom of all pages to make the documents (Quote, invoice …) well design and presentable as printed on real letter head

is there any option to add a digital signature

Put this information in the Notes field, where you can specify default text. Or add custom fields, which appear at the bottom.

When you ask about digital signatures, do you mean showing a signature digitally? If so, see [16.4.16] Added ability to use HTML markup in fields. If you mean a digital authentication signature of some type, Manager will not generate one internally.