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I have worked with the data shared by you. Source pdf file, Scaned Document.pdf, contains embedded fonts which are not being rendered correctly in generated image. I have attached the fonts which are being used in this PDF, ScannedDocument_Fonts.zip. Please refer to these fonts with below line of code, while creating a png image.
FolderFontSource source = new FolderFontSource(@"D:\ScannedDocument_Fonts\");
I have attached output png image Scaned Document_out.png for your kind reference.
ScannedDocument_Fonts.zip (57.3 KB)
I hope this will be helpful. Please let us know if you need any further assistance.