i you want to create pdf with txt file with Encoding: Custom to do direct for my past experience i am using this method to do this kind of work

var pdfReader = new PdfReader(filePath);
var pdfDocument = new PdfDocument(pdfReader);
var strategy = new LocationTextExtractionStrategy();

StringBuilder extractedText = new StringBuilder();
for (int i = 1; i <= pdfDocument.GetNumberOfPages(); i++)
{
    var page = pdfDocument.GetPage(i);
    string pageText = PdfTextExtractor.GetTextFromPage(page, strategy);
    extractedText.Append(pageText);
}

pdfReader.Close();
return extractedText.ToString();

with this method if not work then check this site https://api.itextpdf.com/iText/java/7.1.15/index.html?com%2Fitextpdf%2Fio%2Ffont%2FIExtraEncoding.html= this is officialapi from iText as well check this How to get the text position from the pdf page in iText 7