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  3. OCR Hangs on Large PDFs with EnglishFast Secondary Language

OCR Hangs on Large PDFs with EnglishFast Secondary Language

Curtis Chau
Curtis Chau
Updated:

IronOCR can stall mid-processing on large PDFs when OcrLanguage.EnglishFast is added as a secondary language. No exception is thrown and execution never resumes, so the request appears to run forever.

Execution stops at the ocr.Read() call. No exception is thrown and no error is logged.

The stall is triggered by the EnglishFast model when it sits in the secondary language list on Windows. It has been reproduced in our testing across IronOCR 2025.12.3, 2026.1.2, 2026.2.1, and 2026.3.3 through 2026.7.2. Until a fixed version is published, treat any release in that range as potentially affected. Reordering EnglishFast relative to the other secondary languages does not help.

Solution

1. Find the secondary language configuration

Locate any call to AddSecondaryLanguage that passes OcrLanguage.EnglishFast in your OCR setup.

// This line causes the hang on large PDFs
ocr.AddSecondaryLanguage(OcrLanguage.EnglishFast);
// This line causes the hang on large PDFs
ocr.AddSecondaryLanguage(OcrLanguage.EnglishFast);
' This line causes the hang on large PDFs
ocr.AddSecondaryLanguage(OcrLanguage.EnglishFast)
$vbLabelText   $csharpLabel

2. Remove EnglishFast and keep English primary

Delete or comment out the offending call. Set OcrLanguage.English as the primary language so English recognition quality is preserved, and leave your other secondary languages in place, they are unaffected.

// ocr.AddSecondaryLanguage(OcrLanguage.EnglishFast);
var ocr = new IronTesseract
{
    Language = OcrLanguage.English
};
ocr.AddSecondaryLanguage(OcrLanguage.Hindi);
ocr.AddSecondaryLanguage(OcrLanguage.Marathi);
// ocr.AddSecondaryLanguage(OcrLanguage.EnglishFast);
var ocr = new IronTesseract
{
    Language = OcrLanguage.English
};
ocr.AddSecondaryLanguage(OcrLanguage.Hindi);
ocr.AddSecondaryLanguage(OcrLanguage.Marathi);
Imports IronOcr

Dim ocr As New IronTesseract With {
    .Language = OcrLanguage.English
}
ocr.AddSecondaryLanguage(OcrLanguage.Hindi)
ocr.AddSecondaryLanguage(OcrLanguage.Marathi)
$vbLabelText   $csharpLabel

Dropping EnglishFast removes the fast English model, but keeping OcrLanguage.English as the primary language maintains coverage for documents with significant English text.

3. Re-run the affected PDF

Process the document again. It should now complete through all pages without stalling.

WarningThe hang produces no exception and no log entry. Execution simply stops at ocr.Read(), so a stuck request is the only symptom to watch for.

Curtis Chau
Curtis Chau
  Chat with engineering team now
Technical Writer

Curtis Chau holds a Bachelor’s degree in Computer Science (Carleton University) and specializes in front-end development with expertise in Node.js, TypeScript, JavaScript, and React. Passionate about crafting intuitive and aesthetically pleasing user interfaces, Curtis enjoys working with modern frameworks and creating well-structured, visually appealing manuals.

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