With the freeware programme Calibre one can convert PDF to .epub or .pdb in order to use gamebooks properly. Considering that the books concerned are essentially out of copyright (albeit not legally so if the rights of the authors aren't deliberately relinquished), converting the things to various formats can only increase demand, which may enable the authors to claim a small income from the royalties if they so choose
http://calibre-ebook.com/download_windows