replace string in PDF document (ITextSharp or PdfSharp) - pdf

Question

We use non-manage DLL that has a funciton to replace text in PDF document (http://www.debenu.com/docs/pdf_library_reference/ReplaceTag.php).
We are trying to move to managed solution (ITextSharp or PdfSharp).
I know that this question has been asked before and that the answers are "you should not do it" or "it is not easily supported by PDF".
However there exists a solution that works for us and we just need to convert it to C#.
Any ideas how I should approach it?

Answer 1

According to your library reference link, you use the Debenu PDFLibrary function ReplaceTag. According to this Debenu knowledge base article
the ReplaceTag function simply replaces text in the page’s content stream, so for most documents it wouldn’t have any effect. For some simple documents it might be able to replace content, but it really depends on how the PDF was constructed. Essentially it’s the same as doing:
DPL.CombineContentStreams();
string content = DPL.GetContentStreamToString();
DPL.SetPageContentFromString(content.Replace("Moby", "Mary"));
That should be possible with any general purpose PDF library, it definitely is with iText(Sharp):
void VerySimpleReplaceText(string OrigFile, string ResultFile, string origText, string replaceText)
{
using (PdfReader reader = new PdfReader(OrigFile))
{
byte[] contentBytes = reader.GetPageContent(1);
string contentString = PdfEncodings.ConvertToString(contentBytes, PdfObject.TEXT_PDFDOCENCODING);
contentString = contentString.Replace(origText, replaceText);
reader.SetPageContent(1, PdfEncodings.ConvertToBytes(contentString, PdfObject.TEXT_PDFDOCENCODING));
new PdfStamper(reader, new FileStream(ResultFile, FileMode.Create, FileAccess.Write)).Close();
}
}
WARNING: Just like in case of the Debenu function, for most documents this code wouldn’t have any effect or would even be destructive. For some simple documents it might be able to replace content, but it really depends on how the PDF was constructed.
By the way, the Debenu knowledge base article continues:
If you created a PDF using Debenu Quick PDF Library and a standard font then the ReplaceTag function should work – however, for PDFs created with tools that do subsetted fonts or even kerning (where words will be split up) then the search text probably won’t be in the content in a simple format.
So in short, the ReplaceTag function will only work in some limited scenarios and isn’t a function that you can rely on for searching and replacing text.
Thus, if during your move to managed solution you also change the way the source documents are created, chances are that neither the Debenu PDFLibrary function ReplaceTag nor the code above will be able to change the content as desired.

Answer 2

for pdfsharp users heres a somewhat usable function, i copied from my project and it uses an utility method which is consumed by othere methods hence the unused result.
it ignores whitespaces created by Kerning, and therefore may mess up the result (all characters in the same space) depending on the source material
public static void ReplaceTextInPdfPage(PdfPage contentPage, string source, string target)
{
ModifyPdfContentStreams(contentPage, stream =>
{
if (!stream.TryUnfilter())
return false;
var search = string.Join("\\s*", source.Select(c => c.ToString()));
var stringStream = Encoding.Default.GetString(stream.Value, 0, stream.Length);
if (!Regex.IsMatch(stringStream, search))
return false;
stringStream = Regex.Replace(stringStream, search, target);
stream.Value = Encoding.Default.GetBytes(stringStream);
stream.Zip();
return false;
});
}
public static void ModifyPdfContentStreams(PdfPage contentPage,Func<PdfDictionary.PdfStream, bool> Modification)
{
for (var i = 0; i < contentPage.Contents.Elements.Count; i++)
if (Modification(contentPage.Contents.Elements.GetDictionary(i).Stream))
return;
var resources = contentPage.Elements?.GetDictionary("/Resources");
var xObjects = resources?.Elements.GetDictionary("/XObject");
if (xObjects == null)
return;
foreach (var item in xObjects.Elements.Values.OfType<PdfReference>())
{
var stream = (item.Value as PdfDictionary)?.Stream;
if (stream != null)
if (Modification(stream))
return;
}
}