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If the values for FixWidth and Fixheight are provided in code snippet, then the image placed over PDF document are based over information specified in these properties. If you specify the image dimensions information as specified below, the image seems to be identical to its original size.
image.ImageInfo.FixWidth = 50;
image.ImageInfo.FixHeight = 25;
Please take a look over the attached PDF document that I have generated. I think you need to update the formula that you are using to calculate the dimensions information.
In case I can be of any further assistance, please let me know. We apologize for your inconvenience.