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// For complete examples and data files, please go to https://github.com/groupdocs-viewer/GroupDocs.Viewer-for-.NET
FileStream fsSource = new FileStream(StoragePath + DocumentName,
     FileMode.Open, FileAccess.Read);

// Read the source file into a byte array.
byte[] bytes = new byte[fsSource.Length];
int numBytesToRead = (int)fsSource.Length;
int numBytesRead = 0;
while (numBytesToRead > 0)
{
    // Read may return anything from 0 to numBytesToRead.
    int n = fsSource.Read(bytes, numBytesRead, numBytesToRead);

    // Break when the end of the file is reached.
    if (n == 0)
        break;

    numBytesRead += n;
    numBytesToRead -= n;
}
numBytesToRead = bytes.Length;

return fsSource;