When I upload PDF especially a book to document cloud it re downloads it every time .other apps have a option of make the file available offline and only the changes like highlights etc get synchronized across devices .please have a look at how evernote does it .

Let's say I highlighted something in a file on document cloud in iPad and then I have to open it on Android ,then it should automatically sync the changes rather then redownload the while file or even upload the whole file from iPad .and both the files in iPad and Android should have the option to be available offline .

Please have a look at evernote how it does it .