One of the features of the blockchain is 'immutability'. This means that once a transaction has been confirmed by the network, it can not be removed or altered.
Please note that while you can certainly delete the PDF that you have uploaded, we do not let you delete a PDF document once someone has signed it. That's because every signature is a blockchain transaction and that cannot be reversed or deleted from the blockchain itself.
You may have seen the delete button on the site- this is for deleting the pdf. There are some conditions on deleting:
1) You must be the uploader of the document
2) No one else may have signed the document
You can not delete a transaction (eg signing record) from the blockchain.
To delete a document, from the dashboard scroll down, and click delete on the document you want to delete.
If you can not delete the document due to someone else already signing it, you'll get an error message.