We are aware of an issue with some PDF forms created in the most recent version of Acrobat. Currently our website will not accept these forms and provides an error message. We are working on a patch to fix the issue. In the meantime we have the following workaround will will enable you to upload these via the website.
You can do this by flattening the PDF. To do this drag the PDF into Chrome, select "print" and "save as PDF". This will create a flattened version which should work with our service.