I am using Adobe Acrobat XI Pro to work with PDF documents, many of which I download from Google Books and read on my iPad. However, some of the Google Books PDFs are very large and slow to load on the iPad so I have been trying to reduce their size, with a combination of using Acrobat's OCR, save with reduced size and save optimized options. However, with some of the PDF files (and in reality the ones which are hardest to work with) I constantly get this error message: "The PDF document contained image masks that were not downsampled." This means, it seems, that the images' resolution cannot be reduced - which results in the fact that I cannot make the files any smaller.
How can I make it so that Acrobat can reduce the file size? I have tried to extract each page individually, shrink that, and recombine, but it doesn't see to work.