Sciencereading book opened by WKreader，how to convert？

[imeagor]

WKReader is a proprietary document viewer, both an online version and a downloadable offline application, used by the Chinese academic e-book platform ScienceReading.
The WKReader is necessary for users to view and download Digital Rights Management (DRM)-protected e-books provided by the ScienceReading platform, which is associated with the Science Press.
Accessing Content with WKReader
Users typically access content in one of two ways:
Online Viewing: Users can utilize the web-based WKReader directly in their browser to read e-books online.
Offline Viewing: Users can download a specific file format and open it using a dedicated, installable WKReader software for offline access.

the downloaded book (with a pdf extention) has a limited days (15 days) for only reading by WKreader. How to convert it to normal pdf books?

Wkreader link
https://book.sciencereading.cn/WKReader_ForWindows.exe