The provided information seems to be related to Telerik Ui for WPF PdfProcessing libraries and how the file is saved. Please create a check whether the file with the selected name exists before saving the new one. Check also the read/write permission granted to the application.
Also test if the content of the PDF file e.g. watermarks and other settings cause different behavior on saving the file to as an existing file or to a new file.
If you need further help, please open a separate thread in the corresponding product's forum. In your post, specify this thread and elaborate how you generate the PDF content and let us know the results of the above suggested tests.
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