TWebBrowser是一个嵌入式IE示例，如果你不将其保存为物理文件，IE不允许你显示PDF。
因此，要显示它，您应该编写如下内容：

procedure WriteOnWB(EncodedPDFString: WideString);
var
    Bytes: TBytes;
    MS: TMemoryStream;
begin   
    Bytes := TNetEncoding.Base64.DecodeStringToBytes(EncodedPDFString);
    MS := TMemoryStream.Create;
    MS.WriteBuffer(Bytes, Length(Bytes));
    MS.Seek(0, 0);
    MS.SaveToFile('FileName.pdf');

    // Now you can navigate to 'FileName.pdf'
    WebBrowser1.Navigate('FileName.pdf');
end;

另一种解决方案是使用Chromium（正如Olivier建议的），Chromium（因为它是Chrome）允许您通过base64编码字符串显示PDF文件，例如，在iframe标签中（参见answer）：

<iframe src="data:application/pdf;base64,YOUR_BINARY_DATA" height="100%" width="100%"></iframe>

像流一样无文件

procedure FilelessWB2(encodedURL: string);
  var Bytes:TBytes;
      tMS: TMemoryStream;
      WebBrowser1: TWebBrowser; frm1:TForm;
      JPGImage: TJPEGImage;
begin   
  //Bytes:={TNetEncoding.Base64.}{DecodeString}StrToBytes(filetostring(EncodedPDFString));
    
  tMS:= TMemoryStream.Create;
  try
    HttpGet(encodedURL,tmS)
    writeln('debug size: '+itoa((tms.size)));
     { WebBrowser1:= TWebBrowser.create(self);
      MS.WriteBuffer(bytestostr(Bytes),Length(Bytes)); }
    tMS.Seek(0, 0);
     //tMS.SaveToFile(exepath+'FileNametest.jpg');
     //OpenWeb(exepath+'FileNametest.jpg')
    frm1:= TForm.create(self); 
    with frm1 do begin
      setBounds(0,0,500,500)
      with TImage.create(frm1) do begin
        parent:= frm1;
        setBounds(0,0,460,460)
        JPGImage:= TJPEGImage.Create;      
        JPGImage.LoadFromStream(tMS);                
        Picture.Graphic:=(JPGImage);
      end ;
     Show;
    end; 
  finally
    tMS.Free;
    JPGImage.Free;
  end;  
   //Now you can navigate to 'FileName.pdf'
   //WebBrowser1.Navigate(exepath+'FileNametest.pdf');
end;