Thanks for using our products and sharing the sample application with template documents with us.
I tested the scenario and able to notice that the source PDF document you are trying to open have multiple high resolution images, thats why loading and rendering process is too slow. However, during my testing I tried to open a book of 540 pages and it loads in less time then your provided PDF file which has only 8 pages. I think there is no issue in process the only reason which slow this process is high resolution images. Moreover, We can not compare this process of opening the PDF doucment with Adobe Reader because in our process multiple steps are involved. Sorry for the incinvenience.
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Thanks & Regards,