Replied 23 March 2020, 4:11 am EST
Your original form uses a rather rare and specific 'Perms' feature in the document, as described in #12.8.6 of the PDF 2.0 spec. This entry allows to specify certain permissions on the document in addition to the 'normal' document permissions, in particular in your document those permissions prevent form filling if the original form has been modified (as is the case here).
The interesting thing about those permissions is that they are not enforced by any encryption, and are ignored by all PDF viewers that I tried except the Acrobat Reader. They are even ignored by Acrobat Pro.
Unfortunately, the current version of GcPdf does not allow to clear those permissions. We have investigated this and have added a method to clear them. This will be added in the next hotfix which should become available in a week or so. I will post an update here, till then you can use e.g. our GcPdfViewer for testing:https://www.grapecity.com/documents-api-pdf/demos/gc-pdf-viewer/viewer-edit-all/pdf-cs
(Click the 'open file' icon in the top left.)
I will post and update here when the version that allows clearing the permissions is available.