Wow. That's a big file.
As an aside, I have noticed that some .pdf documents have a large file size even when the document itself may not be too large (not many pages). I can only assume that arises by the way the .pdf is created (which software is used).
If I had nothing else to do, plus lots of internet resources, as well as the necessary .pdf software (all of which are non-starters at the moment), I would have a go at loading each manual in turn, then saving again in a reduced format - a bit like can be done by reducing the "quality" when saving files to .jpg from PhotoShop (for example).
Does anyone fancy having a go?
