I am using demo version of PD4ML.
So, I want to ask that
<link rel="stylesheet" type="text/css" href="https://validPathToCSS" />
In head tag not working.
so does this thing work properly in full version?

There is no functional differences in full and demo versions except the demo version watermarks the resulting documents.

In general the tag should work. But:
- HTTPS is not supported if PD4ML is deployed to Weblogic or WebSphere, as the application servers implement proprietary SSL level, not derived from Java's SSL implementation. Solution: to implement a custom resource loader, utilizes platform specific SSL classes. Or even better - use HTTP instead of HTTPS.

- Make sure the valid path is valid not only from browser, but also from the server where PD4ML deployed to. A usual situation: the server cannot resolve host name etc. Try to enable PD4ML debug output and inspect server's log for errors.

Actually in the html page images from same HTTPS Server is working fine but CSS link from the same server is not working.

As HTTPS server we are using Apache 2.0

Please advice for the same.

ok. Try to switch debug info on (pd4ml.enableDebugInfo() or <pd4ml:transform ... debug="true">) and check server's log if it tries to load the stylesheet at all.

Thank you for your support.