How can I determine if a file is a PDF file?

Here is what I use into my NUnit tests, that must validate against multiple versions of PDF generated using Crystal Reports:

public static void CheckIsPDF(byte[] data)
    {
        Assert.IsNotNull(data);
        Assert.Greater(data.Length,4);

        // header 
        Assert.AreEqual(data[0],0x25); // %
        Assert.AreEqual(data[1],0x50); // P
        Assert.AreEqual(data[2],0x44); // D
        Assert.AreEqual(data[3],0x46); // F
        Assert.AreEqual(data[4],0x2D); // -

        if(data[5]==0x31 && data[6]==0x2E && data[7]==0x33) // version is 1.3 ?
        {                  
            // file terminator
            Assert.AreEqual(data[data.Length-7],0x25); // %
            Assert.AreEqual(data[data.Length-6],0x25); // %
            Assert.AreEqual(data[data.Length-5],0x45); // E
            Assert.AreEqual(data[data.Length-4],0x4F); // O
            Assert.AreEqual(data[data.Length-3],0x46); // F
            Assert.AreEqual(data[data.Length-2],0x20); // SPACE
            Assert.AreEqual(data[data.Length-1],0x0A); // EOL
            return;
        }

        if(data[5]==0x31 && data[6]==0x2E && data[7]==0x34) // version is 1.4 ?
        {
            // file terminator
            Assert.AreEqual(data[data.Length-6],0x25); // %
            Assert.AreEqual(data[data.Length-5],0x25); // %
            Assert.AreEqual(data[data.Length-4],0x45); // E
            Assert.AreEqual(data[data.Length-3],0x4F); // O
            Assert.AreEqual(data[data.Length-2],0x46); // F
            Assert.AreEqual(data[data.Length-1],0x0A); // EOL
            return;
        }

        Assert.Fail("Unsupported file format");
    }

Since you use PDFBox you can simply do:

PDDocument.load(file);

It'll fail with an Exception if the PDF is corrupted etc.

If it succeeds you can also check if the PDF is encrypted using .isEncrypted()

you can find out the mime type of a file (or byte array), so you dont dumbly rely on the extension. I do it with aperture's MimeExtractor (http://aperture.sourceforge.net/) or I saw some days ago a library just for that (http://sourceforge.net/projects/mime-util)

I use aperture to extract text from a variety of files, not only pdf, but have to tweak thinks for pdfs for example (aperture uses pdfbox, but i added another library as fallback when pdfbox fails)