I guess the topic is not connected to the product (Oracle SQL Developer).
And if I right understood you are truing to build context help in "PL/SQL Developer", then bellow info would be useful.
Excerpt from HTML Manual of the product:
Step 2 - Download the HTML documentation for your Oracle Server version.
Oracle Server version File size Oracle 12.1 Library 958 MB Oracle 11.2 Library 826 MB Oracle 11.1 Library 672 MB Oracle 10.2 Library 446 MB Oracle 10.1 Library 257 MB Oracle 9.2 Library 209 MB Oracle 9.0 Library 210 MB
Step 3 - Unzip the documentation library into an appropriate directory.
Step 4 - Start PL/SQL Developer's HTML Help setup and point to this directory.
PS Stop using PL/SQL Developer
Oracle SQL Developer is quite useful and customizable and it's FREE