You are probably best off into breaking this down into components:
1. "receive pdf file sent from Angular"
looks to me like you want to create a POST request from Angular. So search for that and you will find articles similar to this
AngularJS Http Post Method ($http.post) with Parameters Example - Tutlane
2. ... through web API
looks like you want this POST request to be processed in Web API as a file upload. Then search for just that and you will find something like
Uploading a File in ASP.Net Web API
Now the way I would do this would be to create the Web API file upload component first and use a simple web page with file upload on it to test it out and make sure it is working; and then work on the Angular component to replicate that html form