问题可能是由于在PDF转换期间，某些Bootstrap类名被解析器识别为无效字符而导致。解决这个问题的方法是使用iTextSharp库中的CSSResolver类来显式地解析Bootstrap中的相应类。

以下是示例代码：

// Load the HTML file from which you want to generate the PDF
string html = File.ReadAllText(Server.MapPath("~/Views/Home/Index.html"));

// Create an instance of iTextSharp Document class
Document document = new Document();

// Create an instance of the iTextSharp PDFWriter class
PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(Server.MapPath("~/Content/Output.pdf"), FileMode.Create));

// Open the document
document.Open();

// Create an instance of the HTMLWorker class to parse the HTML
HTMLWorker worker = new HTMLWorker(document);

// Create an instance of the CSSResolver class and parse the CSS
CSSResolver cssResolver = new StyleAttrCSSResolver();
CssFile cssFile = XMLWorkerHelper.GetCSS(new FileStream(Server.MapPath("~/Content/bootstrap.min.css"), FileMode.Open));
cssResolver.AddCss(cssFile);

// Parse the HTML and apply the CSS
worker.SetStyleSheet(cssFile);
worker.Parse(new StringReader(html));

// Close the document and writer
document.Close();
writer.Close();

以上代码使用了iTextSharp库中的CSSResolver和HTMLWorker类，显式地解析了Bootstrap中的样式，并将其应用于HTML转换为PDF的过程中。