pdftoppm should handle rotation (landscape) before applying -scale-to options.
I've been using pdftoppm -scale-to-x 1200, but for landscape documents the resulting image gets a Y dimension of 1200, instead of the X dimension. To fix this, I've simply moved a little code to apply the orientation (swap width and height) a couple lines earlier, before computing the resolution from scaling.
Benefits: -scale-to-x will now uniformly set the width of the resulting image. Caveats: anyone who depends on (or works around) the old behavior will now be upset.