Origin of blurring and colour distortion in monochrome diagrams I am analysing a large number of PDF files from scientific articles, all of which contain images which (I think) were originally monochrome ( black/white). 
The images have been extracted with Apache PDFBox and I am doing image processing on *.png.
The images have severe colour distortion, and blurring which I do not think is due to the extraction process or to subpixel antialising (see https://stackoverflow.com/questions/16242731/reconstructing-line-diagrams-from-subpixel-rendering). Note that the effect is present on both lines and text.

The effect is present in all PNG files though in different amounts. I suspect it comes from the tool/s used to create the PDFs from the original images which 
apply some softening process (antialising). I have analysed this visually and it consists of a narrow plane in HSV with only two colours mixed with a lot of white/grey/black.

I'd like to know what the algorithm is that creates this blurring and colour so that I can reverse it. Traditional sharpening is only partially successful.
I'd be delighted if anyone has code that does this (any language will do).


NOTE: I do not have originals.

UPDATE.
Here are some analyses of the colors with ImageJ ColorInspector3D plugin in YCbCr space

= YCbCr frequencies =
Note the very sharp plane in which the colors lie



and a histogram to show the distribution more clearly. 

(The red/pink is there on the RH sides - not very clear in this screen shot)

I intend to ask another question on how to sharpen this. (This Q is *where" does the effect arise; the next will be how to correct it even if we don't know the origin)
This looks like JPEG artefacts so I guess the image went through a JPEG compression step at one stage and that permanently introduced noise into the image. The red channel is blurred stronger because JPEG additionally applies chroma subsampling.

I suspect the original image was subpixel rendered as you shouldn't really have anything in the chroma channels for a monochrome input image.