@AStelzner In HTML format there is no page width concept, but in PDF or word document the content should fit the page. In your case the long string without whitespaces does not wrap properly and goes outside the page bounds. You can specify cell width explicitly to force the string wrap. For example see the following code:
var doc = new global::Aspose.Words.Document();
var builder = new DocumentBuilder(doc);
builder.PageSetup.Orientation = Orientation.Landscape;
double pageWidth = builder.PageSetup.PageWidth - builder.PageSetup.LeftMargin - builder.PageSetup.RightMargin;
var table = builder.StartTable();
builder.InsertCell();
// Specify width of the cell.
builder.CellFormat.Width = pageWidth;
builder.CellFormat.Borders.Right.Color = Color.FromArgb(204, 119, 119);
builder.CellFormat.Borders.Left.Color = Color.FromArgb(204, 119, 119);
builder.CellFormat.Borders.Top.Color = Color.FromArgb(204, 119, 119);
builder.CellFormat.Borders.Bottom.Color = Color.FromArgb(204, 119, 119);
builder.CellFormat.Shading.BackgroundPatternColor = Color.FromArgb(255, 221, 221);
builder.CellFormat.WrapText = true;
builder.CellFormat.VerticalAlignment = CellVerticalAlignment.Top;
builder.Font.Bold = true;
builder.InsertBreak(BreakType.LineBreak);
builder.Writeln("Folgende Fehler sind aufgetreten:");
builder.InsertBreak(BreakType.LineBreak);
builder.Font.Bold = false;
builder.Write(exceptionAfterTransformUsingCustomNamespaces.Message);
builder.InsertBreak(BreakType.LineBreak);
builder.EndRow();
builder.InsertCell();
// Specify width of the cell.
builder.CellFormat.Width = pageWidth;
builder.CellFormat.Borders.ClearFormatting();
builder.CellFormat.Shading.BackgroundPatternColor = Color.White;
builder.RowFormat.HeightRule = HeightRule.Auto;
builder.Font.Bold = true;
builder.InsertBreak(BreakType.LineBreak);
builder.Writeln("Original XML:");
builder.InsertBreak(BreakType.LineBreak);
builder.Font.Bold = false;
builder.Write(inputXml);
builder.EndRow();
builder.EndTable();
table.AutoFit(AutoFitBehavior.FixedColumnWidths);
doc.Save(@"C:\Temp\out.html", new Aspose.Words.Saving.HtmlSaveOptions() { PrettyFormat = true});
doc.Save(@"C:\Temp\out.pdf", SaveFormat.Pdf);
Also, as Tahir mentioned you do not need Aspose.PDF in this case to save the document as PDF, you can save the created document directly to PDF using Aspose.Words.