You can sign a document with Digital Signature Certificate (DSC) issued by a Certification Authority (CA) to make it a valid legal document as per laws of a country.
You can sign an existing PDF document or you can create a word document then save it as a PDF. Then you can digitally sign the PDF document using DSC. A digitally signed document becomes a legal document as per policies of the country.
For being a legal signature, the signature should be purchased from a CA. There will be a number of CA in your country who will be authorized for issuing DSC.
Normally, you will buy a DSC as a USB token. Insert the USB token in your PC, use Adobe Reader or other PDF readers to sign the PDF document using your Digital Signature stored on a USB token.
You can lock the Document to prevent any further changes to the document.
Useful Links used in video:
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