Hi May Zin Linn,
Instead of ticking those two checkboxes, what you should do is make sure your user has the required role (namely Html2PdfAdmin from HtmlToPdfConverter). The change Amit suggested means that anyone (including users not logged in) can change the settings of the PDF generation, which is a big security issue.
Regarding your second question, since you can open the HtmlToPdfConverter, there are two screens called HowToImage and HowToPDF that are examples of how you can use the tool to generate either an image or a PDF from a URL.You can examine how they are implemented in order to do the same for your own PDF generation needs.
Please let us know if you face any extra difficulties