OCR that adds generated text to the original pdf and djvu files?

Question

My OS is Ubuntu.

I found there are some applications can OCR a pdf or djvu file, generating another text file.

But I was wondering how to add the OCRed text onto the original pdf or djvu files, to make it text-selectable in original pdf or djvu files, as Adobe Acrobat does on Windows?

Answer 1

For PDF there is pdfsandwich

pdfsandwich generates "sandwich" OCR pdf files, i.e. pdf files which contain only images (no text) will be processed by optical character recognition (OCR) and the text will be added to each page invisibly "behind" the images.