Hi!
In my experience, there is no built-in editor in Microsoft Teams if you want to edit PDF in Microsoft Teams. The only format we can edit it in Teams are Word, Powerpoint, Excel and Visio files. If your format is pdf, we recommend you convert it to words and then you can add comments and edit it in Microsoft Teams.
If the workaround is not helpful to you, we recommend you give your feedback in Teams UserVoice. (https://microsoftteams.uservoice.com/)Microsoft will always focus on customer’s feedback and experience, some new features would be added to the services based on customers' feedback in the future, and your good ideas will be very helpful for them to improve the service.