Here is the report from General Miles of the murder of the five Miniconjou headmen in December 1876 by his Crow scouts. It is a PDF document (as usual) and honestly, the image quality is not world-shaking but as I told Henri, it is a fine line to walk between getting the image too light to read to hide the blemishes from the original document and dark enough to read with a few blemishes. Any Photoshop or other graphics application expert is welcome to come to the rescue.
Having read through hundreds of archival documents, I can tell you that the one you identify (http://www.historicalresources.net/121776_Miles_DeptDak.pdf) is among the MOST legible I've seen. It's very easy to read -- no PhotoShopping is required.
I suspect that perhaps English isn't your first language, which makes script like this challenging. If that's the case, I suggest that you ask a native English speaking person to read it for you or transcribe it. (It is also helpful to look at the pdf file at higher magnification, e.g. 200%.)