If you mean you accidentally deleted the core plugin Annotate PDF (assignfeedback_editpdf) then you should be able to reinstall this by downloading a copy of the source code for the same Moodle version and copying the folder mod/assign/feedback/editpdf into your site's source code folder. When you log in as an admin you should be prompted to install this plugin and should get the functionality back.
Assignment
Assignment feedback: pdf annotation
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