The first thing I would try is running the Test Server Requirements action from the plugins page to potentially narrow down if this is an issue on the server, or with something in the PDF you're trying to create. If all the tests pass, then I would start looking at the assets in the HTML you're converting to see if any come from other domains which may be having SSL issues.
I hope this helps point you in the right direction - let me know if you have any other questions!
Thank you for your reply.
Unfortunately I didn't find anything for solution yet.
The server req. is fine, the plugin is working from the back-end for every examples.
The problem is popup when I use URL as attachment.
there is not many related help on the net. I purchased a new ssl cert. (by trying to not use self-signed ssl) but no luck.
How can I get the latest wkhtmltopdf-0.12.6-1.centos7.x86_64 file? On the official website there is only rpm file.
Sorry to hear you haven't had any luck yet.
I was able to extract this file from the RPM, I believe it should be the wkhtmltopdf binary for CentOS 7: https://interactivetools.com/uploads/wkhtmltox-0.12.6-1.centos7.x86_64
Let me know if that helps!