DOD is constant during complete Scrum Implementation
Is Definition of Done same for all the PBIs or increments of a particular product?
What is "Definition of Done" ? How does it help complete a PBI ? Once you know answers to these, you would know :-)
Yep, DoD remains same for all PBIs within in a Sprint.
Scrum team keeps refining (tightening) their DoD during the Sprint Retrospective. Changing DoD during the Sprint may impact the forecasted work.
The Definition of Done ought to apply to an increment since that is the artifact which is subject to potential release. This may include various levels of done, one level which is commonly applied being acceptance criteria for user stories. A team should try to improve their DoD continually.