Apparently there is something wrong with a pdf that I need to fill & sign, some of the fields of the form cannot be completed, any idea how to solve this ?
If you are still experiencing this issue, please check if its an issue with a particular PDF file or all the PDFs? Please try with a different PDF file and check.
Have you got the form from a different user, if yes, please ask him to recreate the form using Acrobat Pro and check if that helps.
The "Reader-enabling" feature comes from the original form, and it is essentially a digital signature. It is specific for the free PDF viewer product by Adobe (aka Adobe (Acrobat) Reader).
The problem is that only Adobe products are able to (re)save such documents without breaking the seal of the digital signature.
So, what happened in your case is that the some PDF viewer broke the seal, and that's it. Using Acrobat, you could save a copy of the form, and then continue filling it out. Otherwise, you will be better off to start again, but stay with Adobe PDF viewers all the time working with the form, or with another viewer, again all the time you are working with the form.