We want to add our company's watermark to pdf files (that are uploaded by users).
We found a couple PHP libraries for manipulating PDFs. We're currently using FPDF. It appears fairly straight-forward to add the watermark, but we're struggling to figure out how to remove it.
Is this a limitation of this library? Or is there a more general issue with PDFs that makes this difficult?
I used for basic manipulation FPDF, but once I tried a little bit the java itext library it's pretty awesome. And really I recommend to you further reading iText in action. Just give a try through the vogella tutorial. The time you take for this will be really improve yourself.
Specially you can use the PdfStamper for this purpose. I can share a basic usage in https://github.com/web20opensource/stamper
In something similar way like adding a new layer to a image, it's the same way for manipulation pdfs.... you're adding a new layer. And as the same happens in images you can't remove a layer after you have merged all the layers.