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Read or refuse a document that reaches a limit

The reader bounds what a file may make it hold, and a large but sound document — a large-format scan, a file saved incrementally a thousand times — can reach one of those bounds. This guide shows how to tell, and then how to read the whole document, or refuse it instead.

Tell whether a document reached a limit​

A document that reached a limit was read in part, and says so in its diagnostics, under a code that starts with limit.. The message names the property that lifts it:

Warning limit.decoded-stream at 48213: The /FlateDecode data decodes to more than 256 MB; decoding stopped
there. Raise PdfReaderLimits.MaxDecodedStreamLength to read past it.
foreach (var entry in document.Diagnostics)
{
    if (entry.Code.StartsWith("limit.", StringComparison.Ordinal))
    {
        Console.WriteLine(entry);
    }
}

Reading is lazy: a stream reaches MaxDecodedStreamLength when it is decoded, so look after decoding, not only after opening.

Raise the limit the report names​

The limits are an immutable record on PdfReaderOptions. Raise the one the report names, and open the document again:

using AdCodicem.Pdf.Documents;

var options = new PdfReaderOptions
{
    Limits = PdfReaderLimits.Default with { MaxDecodedStreamLength = 512 * 1024 * 1024 },
};

using var document = PdfDocument.Open("topographic-map.pdf", options);

Every stream of that document decodes under its limits, however long after opening you decode it.

Read documents you trust without limits​

For documents your own application produced, lift every limit at once:

var trusted = new PdfReaderOptions { Limits = PdfReaderLimits.Unbounded };

Never use it for uploads: under it, a file can make the reader hold whatever it asks for, up to about 2 GB per stream or object.

Refuse the document instead​

To refuse a document rather than read part of it, set ThrowOnLimit, and reaching a limit throws a PdfLimitExceededException instead of warning:

using AdCodicem.Pdf.Diagnostics;

var options = new PdfReaderOptions { ThrowOnLimit = true };

try
{
    using var document = PdfDocument.Open(path, options);
    Process(document);
}
catch (PdfLimitExceededException exception)
{
    // exception.Code      "limit.decoded-stream"
    // exception.LimitName "MaxDecodedStreamLength"
    // exception.Limit     268435456
    // exception.Position  the byte offset where the limit was reached
    Reject(path, exception.Message);
}

Put the try around everything that uses the document, not only around Open: the exception comes from whichever operation reaches the limit — Open while it indexes the file, GetObject or resolving a reference when an object is too long, Decode when a stream decodes too far, Validate as it reads.

See also​

  • Reader limits, in the reference: every limit, its default and its code, and the bounds that cannot be lifted.
  • Reader limits, explained: why the guards are on by default.