iText7 PdfTextExtractor.GetTextFromPage "'StandardEncoding' is not a supported encoding name." - itext7

Question

I have a method in our software that pulls the text from a PDF, from a scan or text generated.
I usually try the GetTextFromPage() method first. If it doesn't return text, then I move onto OCR'ing the page.
I have a particular 6 page PDF with the first three pages being a scanned document, and the last two being a form.
On this PDF I'm getting an error that I can't figure out how to resolve.
'StandardEncoding' is not a supported encoding name. For information on defining a custom encoding, see the documentation for the Encoding.RegisterProvider method.
Parameter name: name
at System.Globalization.EncodingTable.internalGetCodePageFromName(String name)
at System.Globalization.EncodingTable.GetCodePageFromName(String name)
at iText.IO.Util.IanaEncodings.GetEncodingEncoding(String name)
at iText.IO.Util.EncodingUtil.ConvertToBytes(Char[] chars, String encoding)
at iText.IO.Font.PdfEncodings.ConvertToBytes(String text, String encoding)
at iText.IO.Font.FontEncoding.FillNamedEncoding()
at iText.IO.Font.FontEncoding.CreateFontEncoding(String baseEncoding)
at iText.Kernel.Font.PdfType1Font..ctor(PdfDictionary fontDictionary)
at iText.Kernel.Font.PdfFontFactory.CreateFont(PdfDictionary fontDictionary)
at iText.Kernel.Pdf.Canvas.Parser.PdfCanvasProcessor.GetFont(PdfDictionary fontDict)
at iText.Kernel.Pdf.Canvas.Parser.PdfCanvasProcessor.SetTextFontOperator.Invoke(PdfCanvasProcessor processor, PdfLiteral operator, IList`1 operands)
at iText.Kernel.Pdf.Canvas.Parser.PdfCanvasProcessor.InvokeOperator(PdfLiteral operator, IList`1 operands)
at iText.Kernel.Pdf.Canvas.Parser.PdfCanvasProcessor.ProcessContent(Byte[] contentBytes, PdfResources resources)
at iText.Kernel.Pdf.Canvas.Parser.PdfTextExtractor.GetTextFromPage(PdfPage page, ITextExtractionStrategy strategy, IDictionary`2 additionalContentOperators)
at iText.Kernel.Pdf.Canvas.Parser.PdfTextExtractor.GetTextFromPage(PdfPage page)
at EFR.OCR.OCR.ExtractTextFromPDF(FileInfo fileInfo, Int32 StartingPage, Int32 NumberOfPages) in P:\Cloud\Dropbox\EF Recovery\OCRTest\EFR.OCR\OCR.vb:line 113
I've processed many PDFs through my code, some text, some scans, some mixed together. Some had forms... This is the first time that I've had this error.
Here's a snippet of my code...
Using reader As New iText.Kernel.Pdf.PdfReader(fileInfo.FullName)
reader.SetUnethicalReading(True)
Using sourceDoc As New iText.Kernel.Pdf.PdfDocument(reader)
If NumberOfPages = 0 Then NumberOfPages = sourceDoc.GetNumberOfPages
For i As Integer = StartingPage To StartingPage + NumberOfPages - 1
Dim pageText As String = ""
Try
pageText = iText.Kernel.Pdf.Canvas.Parser.PdfTextExtractor.GetTextFromPage(sourceDoc.GetPage(i))
Catch ex As Exception
OCRLog.Log($"Error attempting to extract text from page {i}. {ex.ToString}")
End Try
If pageText = "" Then
'extract this page
Dim results As OCRResults = ExtractTextFromPDFImagePage(fileInfo.FullName, i)
pageText = results.Text
pageItems.Add(New OCRResults.PagesClass(results.Accuracy, True, pageText))
Else
pageItems.Add(New OCRResults.PagesClass(100, False, pageText))
End If
stringBuilder.Append(pageText)
Next
Return New OCRResults(stringBuilder.ToString, pageItems)
End Using
End Using
Any ideas?

Answer 1

There is an error in the PDF, just as indicated by the error text "'StandardEncoding' is not a supported encoding name.".
The fonts on the page you shared use the name StandardEncoding in their Encoding entries. This is not a valid name here. According to the specification ISO 32000-1 the only valid values here are MacRomanEncoding, MacExpertEncoding, and WinAnsiEncoding, see Table 111 – Entries in a Type 1 font dictionary – and Table 114 – Entries in an encoding dictionary.
Adobe Preflight also complains about these names when checking for syntax errors:
An unexpected value is associated with the key
Key: BaseEncoding
Value: /StandardEncoding
Type: CosName
Formal Representation: Encoding
Cos ID: 38
Traversal Path: ->Pages->Kids->[0]->Resources->Font->WARSP->Encoding
An unexpected value is associated with the key
Key: Encoding
Value: /StandardEncoding
Type: CosName
Formal Representation: Font.FontType1
Cos ID: 27
Traversal Path: ->Pages->Kids->[0]->Resources->Font->Arial,Bold
An unexpected value is associated with the key
Key: BaseEncoding
Value: /StandardEncoding
Type: CosName
Formal Representation: Encoding
Cos ID: 22
Traversal Path: ->Pages->Kids->[0]->Resources->Font->Arial->Encoding
An unexpected value is associated with the key
Key: BaseEncoding
Value: /StandardEncoding
Type: CosName
Formal Representation: Encoding
Cos ID: 19
Traversal Path: ->Pages->Kids->[0]->Resources->Font->ARROW->Encoding
(Excerpt from a preflight report for your shared PDF)
In spite of StandardEncoding not being a valid name here, the PDF specification knows a "Standard Encoding", see Annex D of ISO 32000-1. Most likely your document attempts to refer to that encoding at the locations outlined above.
If you need to extract text from the document in question, therefore, you may want to follow the recommendation of the error message:
For information on defining a custom encoding, see the documentation for the Encoding.RegisterProvider method.
The Encoding class here is the one in System.Text.
To extract the text from your PDF, therefore, it should suffice to implement an EncodingProvider that for the name StandardEncoding provides an Encoding instance according to the information from the STD column of the table in Annex D.2 – Latin Character Set and Encodings – of ISO 32000-1.