PDF Check in a filestream

FileCheck.cs

using System;
using System.IO;
using System.Text;

namespace Example
{
    public class FileCheck
    {
        /// <summary>
        /// Determines whether the specified file stream is a valid PDF.
        /// </summary>
        /// <param name="fileStream">The file stream.</param>
        /// <returns><c>true</c> when the stream has a start and end according to the PDF specification; <c>false</c> otherwise.</returns>
        /// <remarks>See http://wwwimages.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/PDF32000_2008.pdf for the PDF specification.
        /// Also note that the check executed is quite generic.
        /// </remarks>
        internal static bool IsValidPdf(Stream fileStream)
        {
            if (fileStream.Length < 12)
            {
                return false; // as we cannot have both the header and the trailer contained in the file
            }
            fileStream.Seek(0, SeekOrigin.Begin);
            byte[] buffer = new byte[7];
            fileStream.Read(buffer, 0, 5);
            //PDF with BOMs are not valid
            bool headerCorrect = Encoding.ASCII.GetString(buffer).StartsWith("%PDF-"); 
            fileStream.Seek(-7, SeekOrigin.End);
            fileStream.Read(buffer, 0, 7);
            //Validate the last line is %%EOF with contains, as there can be line endings after the EOF, which can be \r or \n or both 
            bool trailerCorrect = Encoding.ASCII.GetString(buffer).Contains("%%EOF"); 
            fileStream.Seek(0, SeekOrigin.Begin);
            return headerCorrect && trailerCorrect;
        }
    }
}