The answer of EndNote-Support on my request:
Thank you for contacting Clarivate Analytics Technical Support.
I could certainly understand your situation and I apologize for the trouble you are experiencing.
I am sorry to inform you that we have an ongoing issue with EndNote X8 that searching within the PDF crashes EndNote. We have already reported this to our developers and they are diligently working on it to resolve this issue. Therefore, I am going to forward your comments to our developers in addition to the existing issue. However, I am sorry that I am unable to provide you the exact time frame when this will be resolved. I shall definitely get back to you if I receive any updates from them.
In the mean time, you can try opening the PDF in any third party PDF Viewer application as a workaround.
Thank you for your understanding and patience!
Regards,