I will add this as an answer, because it answers the question somehow and it is long and a bit complicated. The short answer is: no, it is not possible. Once you can read the PDF on your screen, it means that there's all the information and the PDF can be edited. Adobe programs of course do not offer such functionality. But if you want the PDF to be editable, you can always use ImageMagick:
convert -density 600 myfile.pdf myfile.png
Afterwards, since the quality is very high, any sufficiently good OCR will be able to convert it back to text. And there are surely other possibilites. As was mentioned by the other people, you can decode the PDF to PS (which is possible because to some extent, this is what the PDF reader has to do) and then convert back, and you have an editable PDF file.
Conclusion: You can protect the PDF from edits by "normal" users, but you cannot protect it from edits by people who know what they are doing.