keyword, tags, labels, full-text indexing, semantic indexing are all technologies which could help me to easily organize all my PDFs

The key problem with these technologies is the fact that nobody really has the discipline to label (or tag or give keywords). A good semantic search engine with good common dictionaries could greatly increase the findability of my files.

For those of us who have the discipline to tag or label the files virtual folders in which certain combination of tags or labels are used as filters are certainly the way to go.

"Physical" folders with files residing only in one place are certainly not the way to go into the future.