PO issues

The word "estimate" is not used in any form in the current revision of the Scrum Guide.  The guide does use the word "size" twice both in the section that describes the Product Backlog.

Product Backlog refinement is the act of breaking down and further defining Product Backlog items into smaller more precise items. This is an ongoing activity to add details, such as a description, order, and size. Attributes often vary with the domain of work.

The Developers who will be doing the work are responsible for the sizing. The Product Owner may influence the Developers by helping them understand and select trade-offs.

The developers are responsible for sizing the work related to Product Backlog Items as they are the ones that will do the work.  

Your situation is an opportunity for the Scrum Master to help the Product Owner understand the accountabilities and responsibilities that everyone in the team has.

I also feel like it would be beneficial to remind everyone on the Scrum Team that estimates are made upon information known at the time they are made. They are made to help the Developers in determining what body of work could potentially be done during a Sprint timebox.  Beyond that, they have no relevance at all.  If the Product Owner feels that they have information that is not known to the Developers, then the Product Owner should be sharing it. But it is up to the Developers to decide if that information impacts their sizing decisions. 

I am facing this issue where the PO has technical knowledge and is questioning often with the estimates the Dev team comesup with 

Why? Does the PO have reason to suspect that the estimates might be too low?