Rename files downloaded from Packt via LinqPad

Rename.cs

void Main()
{
    var dir = @"Y:\Downloads\My Books\";
    var filepaths = Directory.GetFiles(dir, "*.pdf", SearchOption.TopDirectoryOnly);
    var pattern = @"^\d{13}\-(\w+)\.pdf$";
    var regex = new Regex(pattern);

    foreach (var filepath in filepaths)
    {
        var filename = Path.GetFileName(filepath);
        if (!regex.IsMatch(filename)) continue;
        
        var matches = regex.Matches(filename);
        var fnbuilder = new StringBuilder();

        foreach (var part in matches[0].Groups[1].Value.ToLower().Split('_'))
        {
            fnbuilder.Append(part.First().ToString().ToUpper() + part.Substring(1) + "_");
        }
        
        var newfilename = fnbuilder.ToString().TrimEnd('_') + ".pdf";
        var newfilepath = Path.Combine(dir, newfilename);

        if (File.Exists(newfilepath))
        {
            Console.WriteLine(newfilepath);
        }
        else
        {
            File.Move(filepath, newfilepath);
        }
    }
    
}

Example

Renames 9781788628815-UNDERSTANDING_SOFTWARE.pdf to Understanding_Software.pdf.