I am building a ui, where i want to extract pantone shades/colors from a pdf file. Can't find any documentation for the same.
Can you clarify what UI you are using? Is it a website? Adobe Acrobat? or some other product.
Have you studied the PDF Reference Manual? This is a must, the APIs you might use (Cos or PDFEdit according to your preference) do not target separation information but give you access to colour space definitions. PDF has 11 kinds of colour space, 2 of which are spot colour (and a third might embed references to spot). This assumes your Pantones are spots. If it was the pantone process guide there is nothing in the PDF related to the swatches used to generate CMYK.
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If you are doing this in Acrobat, then you'll need to write a plug-in using C++ and the Acrobat SDK. If you want to do this outside of Acrobat, then you'll need a library with sufficient granularity to expose the color resources. For example, iText, PDFBox, PDFLib, or the official PDF library licensed by Datalogics. You can already view this information in several ways. For example, load the PDF into InDesign, although this doesn't work for all PDFs. There several 3rd party tools for printing that specifically show this type of info. PDF CanOpener (COS Level Editor for PDF ) is a 3rd party tool for exposing the raw COS structure of a PDF, so you can see exactly how the color resources are structured. This tool is indispensable when you are writing tools that operate at this level.
Regardless of what you do you'll need to read the PDF Specification. Google it.