How can I load pdf file in richtext bx editor in asp.net mvc

If you mean that you want to get the text from a searchable PDF file and show the text in RichText box, you can do that using LEADTOOLS PDF Document class and parse the PDF objects. Then get the text objects only, and append them in the RitchText box. For more information see the following link:
https://www.leadtools.com/help/leadtools/v19m/dh/pdf/pdfobject.html

The code is very simple:
/**********************************/
using Leadtools;
using Leadtools.Codecs;
using Leadtools.Pdf;

StringBuilder sb;
sb = new StringBuilder();
PDFDocument document = new PDFDocument(@"PDFs\leadtools.pdf");
PDFParsePagesOptions options = PDFParsePagesOptions.Objects;
document.ParsePages(options, 1, -1);
foreach (PDFDocumentPage page in document.Pages)
{
Console.WriteLine("Page #" + page.PageNumber.ToString() + " Started");
foreach (PDFObject obj in page.Objects)
{
if (obj.ObjectType == PDFObjectType.Text)
{
if (obj.TextProperties.IsEndOfLine)
sb.AppendLine(obj.Code.ToString());
else
sb.Append(obj.Code);
}
}
Console.WriteLine("Page #" + page.PageNumber.ToString() + " finished");
}
this.Text = "Finished";