[tomsem]

If your client insists on PDF, at a minimum you'll want to create one that is formatted for 6" screen (as well as a 100% scale version for desktop/laptop consumption), by changing the page size in the editing program (MS Word, InDesign or whatever) and making appropriate layout adjustments before creating the PDF. At least then it will not get scaled down and will remain readable. As others point out, ereaders vary in terms of support for specific PDF features like PDF bookmarks, PDF Reflow, but at least they all open and turn pages, etc.

Note that you cannot sell PDFs through Amazon, Apple, Kobo, B&N etc. But if this is just to make it available on a web site or via email distribution, it would be a lot simpler and cheaper than creating mobi and epub files and all of the associated testing that (should) involve.