string mypath = "Document.docx";
Word.Application wordApp = Application;
wordApp.Documents.Open(mypath);
this.Application.ActiveDocument.Range(1,2).PageSetup.PaperSize = Word.WdPaperSize.wdPaperLetter;

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Document doc = new Document( "Section.ModifyPageSetupInAllSections.doc");

// It is important to understand that a document can contain many sections and each

// section has its own page setup. In this case we want to modify them all.
foreach (Section section in doc)
section.PageSetup.PaperSize = PaperSize.Letter;
doc.Save( "Section.ModifyPageSetupInAllSections Out.doc");

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