I have a book scan as a pdf. When I open it with Adobe Reader, it looks like grayscale. When I open it with IrfanView, it looks like grayscale, and the Information option tells me that the image is actually 24 bit (I don't know if this is the real bit depth of the image embedded in the pdf or if IrfanView assigns the maximal depth when opening a pdf as image).

I want to OCR the scan with OmniPage SE. It doesn't read PDF, so I decided to use ImageMagick to convert the file to PNG first. But no matter what I try, the output is always monochrome and practically unreadable.

I tried different conversion lines, with different depth, density and resize values, but it didn't help. What you see was made with the options convert testfile.pdf -density 600x600 -depth 8 PNG:testfile.png.

Any idea what causes the problem?

Edit: To make it clear, the output looks like this for any value of -density, -depth and -resize I have tried. It also looks like that when I use no options at all, as in convert testfile.pdf PNG:testfile.png.