I am guessing this is a long shot, but here goes.

We email a PDF to a client, they print it out, sign it and perhaps annotate it, scan it back in and then email it back to us. So the PDF that comes back is not nicely formatted, but rather a picture saved as a PDF. I need to compare the two PDFs and find the annotations.

Are there any libraries preferably in C# (paid and free) that would be helpful in this?


1 Answer 1


The company I work for has an imaging SDK that could help you with this. The idea I have in mind is as follows:

  1. Load both the original and modified PDF pages as images at the same resolution. The code for that is simple:

    RasterCodecs _codecs = new RasterCodecs();
    RasterImage imageBefore = _codecs.Load(pdf1);
    RasterImage imageAfter = _codecs.Load(pdf2);

  2. Align both images so that corresponding areas have the same coordinates. This part needs some work, but it has already been done in this forum post. You will need to identify certain search keywords at approximate locations when you're designing your program, but after that, the alignment process becomes automated using OCR.

  3. Once you have the 2 images aligned, you can subtract (or XOR) one from the other to obtain areas where they're different. This is done using the CombineFastCommand class

If you would like to try our SDK, we have a fully functioning free evaluation that you can find here. The evaluation comes with free email and online chat support and you can contact support before and during evaluation with any questions you have, including technical questions about our functions and sample code programs.

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