There seems to be a bug in PdfDocumentView control. If window is filled with PdfDocumentView and ZoomMode is set to FitPage. Then if you increase the window size, hence increasing PdfDocumentView's size as well, and then again you set ZoomMode to FitPage. It will use first set ZoomMode size, not the current one.
Set Window size to 400 x 400
Set Zoom to FitPage
Increase Window size to like 800 x 800
Set Zoom to FitPage again (this time it wont work, it will use size of 400 x 400)
USUthandaraja Selva Sundara Kani Syncfusion Team September 2, 2019 06:38 AM
Greetings from Syncfusion.
We were able to reproduce the issue with “PdfDocumentView FitPage does not working properly while resize” in our side. We have forwarded this issue to our development team for further analysis and we will update further details on 04th September 2019.
ASAravindh Sathiyamoorthy Syncfusion Team September 4, 2019 11:02 AM
On further analyzation, we suspect that this issue might have occurred due to defined size given for the PdfDocumentView control. So, this might have not changed the PdfDocumentView size based on the window size. In this case, the size should not be defined for the PdfDocumentView, instead apply Dock value as fill to achieve your requirement. Please find the code snippet for the same from below,
pdfDocumentView.Dock = DockStyle.Fill;
Also, we have created a sample for the same and that can be downloaded from the below location,