Processing Large Multi-Page PDFs with IronOCR

The Problem with OcrInput.LoadPdf()

A common first attempt is to hand the whole file to LoadPdf():

ocrInput.LoadPdf("large.pdf");

ocrInput.LoadPdf("large.pdf");

ocrInput.LoadPdf("large.pdf")

This loads every page at once, which is what triggers the trouble:

• Memory spikes sharply as the entire document enters memory.
• IronOCR's internal imaging system processes all pages simultaneously.
• DPI defaults to 200, which drives memory use higher still when left unset.
• The result can be System.OutOfMemoryException or resource deadlocks.

Solution

Process one page at a time with LoadPdfPage() instead of loading the whole document.

var ocr = new IronTesseract();
var pdfPath = "large.pdf";
using var pdf = IronPdf.PdfDocument.FromFile(pdfPath);
var pageCount = pdf.PageCount;
var textBuilder = new StringBuilder();
for (int i = 0; i < pageCount; i++)
{
    using var input = new OcrInput();
    input.LoadPdfPage(pdfPath, i, 80);
    var result = ocr.Read(input);
    textBuilder.Append(result.Text);
    textBuilder.Append(' '); // Add space between pages
}
Console.WriteLine(textBuilder.ToString().Trim());

var ocr = new IronTesseract();
var pdfPath = "large.pdf";
using var pdf = IronPdf.PdfDocument.FromFile(pdfPath);
var pageCount = pdf.PageCount;
var textBuilder = new StringBuilder();
for (int i = 0; i < pageCount; i++)
{
    using var input = new OcrInput();
    input.LoadPdfPage(pdfPath, i, 80);
    var result = ocr.Read(input);
    textBuilder.Append(result.Text);
    textBuilder.Append(' '); // Add space between pages
}
Console.WriteLine(textBuilder.ToString().Trim());

Imports IronOcr
Imports IronPdf
Imports System.Text

Dim ocr As New IronTesseract()
Dim pdfPath As String = "large.pdf"
Using pdf = PdfDocument.FromFile(pdfPath)
    Dim pageCount As Integer = pdf.PageCount
    Dim textBuilder As New StringBuilder()
    For i As Integer = 0 To pageCount - 1
        Using input As New OcrInput()
            input.LoadPdfPage(pdfPath, i, 80)
            Dim result = ocr.Read(input)
            textBuilder.Append(result.Text)
            textBuilder.Append(" ") ' Add space between pages
        End Using
    Next
    Console.WriteLine(textBuilder.ToString().Trim())
End Using

Compress the PDF First for Image-Heavy Files

If the page-by-page approach still isn't fast or stable enough, compress the PDF with IronPDF's Compress API before handing it to IronOCR. This cuts the amount of image data IronOCR has to process, which pays off most on scanned or image-heavy documents.

Compress the file to a stream, then load that stream straight into OcrInput:

var pdf = PdfDocument.FromFile(pdfPath);
var stream = pdf.CompressPdfToStream(CompressStructTree: true);
var ocrTesseract = new IronTesseract();
using var ocrInput = new OcrInput();
ocrInput.LoadPdfPage(stream, 1);

var pdf = PdfDocument.FromFile(pdfPath);
var stream = pdf.CompressPdfToStream(CompressStructTree: true);
var ocrTesseract = new IronTesseract();
using var ocrInput = new OcrInput();
ocrInput.LoadPdfPage(stream, 1);

Imports IronTesseract

Dim pdf = PdfDocument.FromFile(pdfPath)
Dim stream = pdf.CompressPdfToStream(CompressStructTree:=True)
Dim ocrTesseract = New IronTesseract()
Using ocrInput As New OcrInput()
    ocrInput.LoadPdfPage(stream, 1)
End Using

For PDFs that carry images, lower the JpegQuality during compression to shrink the data further:

var pdf = PdfDocument.FromFile(@"D:\hugePdf.pdf");
var stream = pdf.CompressPdfToStream(JpegQuality: 75, CompressStructTree: true);

var pdf = PdfDocument.FromFile(@"D:\hugePdf.pdf");
var stream = pdf.CompressPdfToStream(JpegQuality: 75, CompressStructTree: true);

Imports System

Dim pdf = PdfDocument.FromFile("D:\hugePdf.pdf")
Dim stream = pdf.CompressPdfToStream(JpegQuality:=75, CompressStructTree:=True)

Debug Tips

• Lower the DPI (80 to 100): when visual quality allows, this cuts memory usage noticeably.
• Avoid reading the entire PDF at once unless the file is very small.
• Dispose objects properly with using statements to release memory early.
• Run OCR in parallel only if the machine has enough memory and CPU headroom; be cautious with large PDFs.

Combining IronOCR with IronPDF (or another PDF utility) lets you process large documents page by page without crashing the application, keeping the workflow stable and scalable in production.