Online Books
Classicly has almost 1800 free novels in its Young Adults collection. These are available for free download in PDF.
BookRix has almost 1000 free ebooks in its Juvenile fiction category. These are available for free reading online.
GoodReads has a listing of free young adult ebooks. Many are available for download at their site.
DigiLibraries has over 2,000 free ebooks in their juvenile fiction category. Most are available for free reading online as well as free download in PDF.
Still looking for a book? Try a google search by typing the title and add .pdf!
For example: Diary of a Wimpy Kid .pdf
Many popular books can be found this way and enjoyed for free!