They also can use and improve it and give these improvements back to the source.
AGPL requires open-sourcing your entire project (authentication, payment platform, etc) just to link to a backend PDF processing library.
AGPL does not have a linking exception.
Might also be a startup idea: AGPL servers as a Service. Someone else runs it for you, you make requests against it.
I think in these cases one should try to understand what the IPR holder had in mind when they picked AGPL and not for example GPL or Apache. Trying to find loopholes might just get you into trouble in future.