For those who are not acquainted with the following site, you may find its large collection of Arabic books in pdf format very useful.
I came across this website just recently and hope to collect a list of sites like it that provide pdfs of books online for free. If anyone has information that would assist in compiling this list, please do share.
Link
Published by Bilal Ali
Bilal Ali is a student of the sacred Islamic sciences with a special interest in Hadith, Hanafi law, and Education. He works currently as a researcher, translator, and instructor in Arabic and Islamic studies at Darul Qasim (Glendale Heights, IL) and serves as Chairman for the Islamically integrated psychotherapy institute, Khalil Center (Lombard, IL).
View more posts
Ṭaḥāwī 
I expect you haven’t heard about 4shared.com :) It has a gigantic collection of Arabic books.
Assalamu Alaikum
Is Fath al Mulhim by Mawlana Shabbir Ahmad Usmani available in PDF online?
i want arabic books/ oslamic
Asalamualaykum…I am one of your brothers in Islam,I highly need your help siriously course of Allah by sending these books to me about Iman,propagation of Islam,pupils traning in Islam.
kindly help me to send them to me through this postar address:Yaqub A. Ganiy Adewale
P.o.Box 819,Iwo,OsunState,Nigeria.