Giving feedback about the paper is a way to show who you are and how much you care about the author.

Suppose, in your opinion,  the author has made a glaring error in logic or has not supported the thesis, or mischaracterized an opposing view, and because you are concerned about hurting their feelings, you don't say anything.

How are you helping them? By letting their paper out into the world?

If you were right about the paper, and didn't tell them, shame on you.

If you were wrong about the paper, that should emerge in the continued dialogue between professionals and now you have a chance to sharpen your own tool. You miss that chance if you don't CRITIQUE THE PAPER, NOT THE PERSON.

If you comment on the paper without regard for the human who offered their vulnerability, their knowledge, their insights, THEMSELVES to you, try remembering to walk a mile in their shoes and ask yourself, before sending, have I been fair? have I been constructive? What is the tone of voice I used?

If you would say things anonymously about the paper in a double blind, but not to their face, that says more about you than about the paper you are critiquing.

Envision the paper as it leaves their hand and lands on a community table of knowledge for consideration. Focus on the paper on the table, not the person who offered it. The paper is not the person; restrict yourself to examining what has been offered. Don't assume you know anything about their feelings or how they might take it. They have offered a piece of academic writing. Your duty is to evaluate it academically, while remembering there is a person on the other end, eventually.

The author has given us all a gift. Respect the gift by giving it your best critique: with support, with care, with your best work.  Respect the author for their gift and vulnerability. The critiques we offer are more important than anything we are likely to write on our own, and we will do a lot more of them than our own writing.

If you are an author, recognize the boundary between your Self and your paper. Be clear about what you are asking for when you offer it for review. If you want self-esteem more than honesty, you'll get both, but not as you might want it.