Try this, should work. But here is another problem u need to delete this file every time after assertion, because test case will always pass, if u downloaded any .pdf file even once. Let me know if it works for u, then we can try to delete file.
string pdfFile = @"c:UsersestcaseDownloads", "*.pdf)";
Console.WriteLine(File.Exists(pdfFile) ? "File exists." : "File does not exist.");
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