Problem with reading PDF document

I have two PDF files that opened fine in Acrobat, and I can page down/up fine without any problem.
My web app uses
FileMode.Append

to open one of the PDF files to merge content from another PDF file. I don't know if it works or not, but no error displayed. And the app finishes running, I try to open the merged PDF file to see if the contents are merged. In Acrobat, the pdf opens with first page successfully, but when I navigate to the next page there is an error with the following message, "There is a problem reading this document (14)".
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