Spire.PDF 9.7.14 では PDF を Word に変換する際に同じフォント名を保持する機能がサポートされています

PdfDocument doc = new PdfDocument();
//新しいページの追加
PdfPageBase page = doc.Pages.Add();
PdfGrid grid = new PdfGrid();
grid.Columns.Add(1);
PdfGridRow headerRow1 = grid.Headers.Add(1)[0];
//HEADER WITHOUT UNCICODE SUPPORT BUT VALID WORD FONT
headerRow1.Style.Font = new PdfTrueTypeFont(new Font("Arial", 11f, FontStyle.Regular), true);
headerRow1.Cells[0].Value = "Spire.PDF for .NET";
headerRow1.Cells[0].StringFormat = new PdfStringFormat(PdfTextAlignment.Center, PdfVerticalAlignment.Middle);
grid.Draw(page, new PointF(0, 10));
//文書をストリームに保存する
MemoryStream stream = new MemoryStream();
doc.SaveToStream(stream, FileFormat.PDF);
stream.Position = 0L;
PdfToWordConverter converter = new PdfToWordConverter(stream);
converter.SaveToDocx(@"out.docx");

PdfDocument doc = new PdfDocument();
doc.LoadFromFile(@"in.pdf");           
PdfSecurityPolicy securityPolicy = new PdfPasswordSecurityPolicy(userPassword, ownerPassword);
securityPolicy.EncryptMetadata = false;
securityPolicy.EncryptionAlgorithm = PdfEncryptionAlgorithm.AES_128;
securityPolicy.DocumentPrivilege = PdfDocumentPrivilege.AllowAll;
securityPolicy.DocumentPrivilege.AllowPrint = false;
doc.Encrypt(securityPolicy);
doc.SaveToFile(@"out.pdf");