scribddown was a great service. It allowed to download only 5 files per IP per day, but it could download files which were not downloadable via other scribd downloaders. Also, some downloaders download some PDFs corrupted, with empty pages, or black pages, or other defects. I used to redownload them via scribddown. They used another script. Unfortunately, they stopped servicing. If you find similar service, please inform.
Some files are not downloadable via downloaders, and they do not have download button on scribd itself.
In this case go to docdownloader.net - it does let issuu downloads and does not let scribd downloads, but it allows full view of scribd files. So, if PDF is made of jpgs, you can select all pages from top to bottom and copy them - jpgs links will be caught by jDownloader (use it, if still not using) - jDownloader will download them as jpgs folder, then you can compile PDF. If PDF is not of jpgs, you still can download via Print -> Save to PDF option, but here again you should highlight the entire book from top to bottom before choosing Print.