Restucture code to wrap optional resources in a using block
The only think that I would do different is that I wouldn't dispose streams inside your FileProcessor classes. Disposing of resources should be done at the same level (or scope) where they were initialized. Unfortunately, even BCL classes like StreamWriter and BinaryWriter dispose underlying streams when disposed, so I cannot claim that this is unexpected behavior either.

IMO, there is nothing wrong with your code. Going out of your way simply to save two lines of code might make your code less readable, so your first version might easily be the simplest one.

Having said that, if you really want to wrap it in a single using block, one idea (and I don't actually find it "better" than yours in any way) might be to wrap all your disposable resources into a single class:

public class InputFiles : IDisposable
{
    private readonly Dictionary&lt;string, Stream&gt; _files 
        = new Dictionary&lt;string, Stream&gt;();

    public Stream GetStream(string type)
    {
        Stream input = null;
        _files.TryGetValue(type, out input);
        return input;
    }

    public InputFiles(string[] args)
    {
        // this is just an idea, you probably don't
        // use the extension to determine the type,
        // but it still seems a bit more general than
        // having strongly typed properties
        foreach (var path in args)
        {
            var ext = Path.GetExtension(path); // pdf or tif?
            _files[ext] = File.OpenRead(path);
        }
    }

    public void Dispose()
    {
        foreach (var stream in _files)
            stream.Value.Dispose();
    }
}


And then dispose the entire object when done:

static void Main(string[] args)
{
    using (var input = new InputFiles(args))
    {
        var pdf = input.GetStream("pdf");
        var tif = input.GetStream("tif");

        // use the appropriate strategy
        // (parsing strategy should not be concerned with
        // disposing of resources)
        Process(pdf, tif);
    }
}