This is the first time I have a bad PM and it's much worse than having a pain in the ass colleague dev. A bad dev will mess his/work project and maybe slow down 1-2 other devs.
But a bad PM will doom the whole project, wasting lots of time of the devs working under him/her. Costing much more company's money.

PM:This task should be ready by next week.
Me : This task will require X weeks time for developing and delivery
PM: What?! That's too long, it's a simple one, should be done in a few days.
Me: **explaining the challenges, limitation, env set up, testing etc. Also because I am a junior so may take more time than experienced dev**
PM: **insist that this is important blah blah**
Me: Understand your points but X days is just too little, I don't want you to blame me for missing the deadline. Either we get a reasonable deadline or you can get more experienced dev to do it faster.
**Knowing well that I have the most experience in this task and other devs are busy with their own tasks**

In the end I have to escalate this argument to more senior manager because both of us won't budge. Not only she agreed to extend the deadline she also assigned a senior dev to help me when I am stuck.

His other mistakes I noticed during my time working under him:
- not consulting senior dev for the approach to the task (thus we have to change the design twice).
- assigning tasks to people without sufficient background (a java dev is being assigned a python task, it's doable but it's going to be faster if we assign to someone with more python experience right?)
I understand that our company is short-staffed, but I begin to wonder if the stress the devs endure is because of that or because of his incompetence.

Next time, I am going to specifically ask not to work under him again.