@MatteoHoch-4475 Thanks for your question.
Is it possible to force the callback url for https://westus2.dev.cognitive.microsoft.com/docs/services/form-recognizer-api-v2-preview/operations/GetAnalyzeFormResult to expire?
No the callback URL cannot be forced to expire currently. It expires 48hrs after the analyze operation of form recognizer. And similar behavior for the Read API.
Are PDF's passed in requests ever stored? Can we disable this?
As part of the analyze or read operation the PDFs are not stored as they are only extracted and processed where the result is available in the form of operation results in the GetAnalyzeFormResult API. As part of training if you have used a storage account for the documents these would be available in the storage account of your choice and you can enable encryption on the storage or delete them after training and the creation of a model. It also allows to enable encryption for any data at rest as part of cognitive services security guidelines.