I don't think Adobe are going to recommend any alternative, just note that alternatives exist. Except for XFA forms, portfolios, and Flash content.
I have the same question.
I need to be able to sign pdf documents on linux. Since version 9 is no more supported, what solution does Adobe recommend for Linux users?
Since you have the same question I have the same answer. Signing PDFs is an entirely public specification from ISO. If there isn't a product, the open source community might create one.
You can try our free PDF reader on Linux, called PDF Studio Viewer. It has advanced search and print options and many viewing tools. It can open encrypted and signed PDFs.