Senior taking credit for my work

I'm working as an APM at a startup for about a year now. I work with another guy who happens to be my senior, PM and both of us report to the director.

Of late, my senior, i.e., the PM has been undermining my work when the two of us are working together, subtly taunting me that what I've done is pretty easy and non impactful business wise. He also disregards the decisions made by me, modifies the PRDs discovered and written by me and mails them as his own.

Eventually, he ends up adding me to the walkthrough calls because I had discovered the features but then concludes everything such that he had owned everything end to end.

Recently, both of us were given a documentation to be done by our manager, i.e., the director. I had completed almost 70% of that document, but in our call with the director he presented it saying that he had written that document and I had made a few changes to it.

This undermines my work and visibility of my capabilities. I don't know how to tackle this situation, my manager is usually busy and leaves it upto this PM to allocate work, which he unfairly uses.

How do I talk to my manager about this. I'm fairly new to corporate, just graduated last year and don't know how to handle all this.

But this thing has been bothering me lately, really looking forward to any advise my fellow grapeviners may have.