Object reference not set to an instance of an object
It's also not specific to PDF, happens if you save as other formats too so there is a problem reading the DOCX.
The sample DOCX file is attached.
Object reference not set to an instance of an object
Document document = new Document();
document.LoadFromFile(@"Capital-Volume-I.docx", FileFormat.Docx);
ToPdfParameterList ps = new ToPdfParameterList();
ps.UsePSCoversion = true;
document.SaveToFile("output.pdf", ps);