I made a desktop application by using PDFbox in Netbeans.Now I wish to develop a web application of same.Can PDFbox be used to make web app using html,php and javascript? I haven't started working on backend yet.
No, unless you use the command line utilities, or develop your own, and then call them from your php / javascript code. See here: https://pdfbox.apache.org/1.8/commandline.html
and here people who call PDFBox from php:
https://github.com/chelmertz/PDFBox-php
http://www.phpclasses.org/package/9028-PHP-Extract-text-from-PDF-documents-using-PDFBox-tool.html