The only thing i noticed when discovering PDFShift a few months ago is, that they include a meta comment that says "Created by PDFShift" (or similar). While this might be okay in most cases, there should be an option (paid?) to remove this information. Not every company (especially those, which are located in the EU) don't want to disclose, that they are using an external service to create their documents (e.g. invoices or other documents, including sensitive informations). Review collected by and hosted on G2.com.