Historically the default for opening PDF documents has been the Adobe Acrobat Reader. Currently, many computers that do not have this reader application installed will open PDF documents in a browser.
General, this is not problematic as browsers will display information within PDF documents correctly. However, fillable PDF forms are not always rendered properly and will not allow entering into form fields. This is a problem with the Microsoft EDGE and Firefox browsers. PDF forms function properly in Google Chrome browser.
Saving files as FDF documents ensures that they will open only in a PDF reader application and not within browsers.
Workaround: Create a PDF document. Rename as an FDF document. This will force the document to be opened in the Acrobat Reader and not in a browser. The Acrobat Reader will correctly interpret the FDF document as being a PDF and will render it appropriately.
A common practice is to notify users and provide a link for them to download the free Acrobat PDF Reader. Download Acrobat Reader.