If you have many PDF files, then you better find a way to organize them on your device. Thus, the software or digital library which you like to use determines which operating system is better for you and hence which device you should pick.
I personally prefer Zotero to organize my PDF files which is only avaiable on Windows. As a result, I bought a 10 inch touch laptop which can be detached from its keyboard. One example is acer switch 10.
If you like Mendeley, then you have three choices, Android, IOS, and Windows. I did not work with Mendeley on IOS. I think between Android and Windows Mendeley works better on Windows.
Consider that the battery on Android and IOS devices last longer than laptops and you can charge these devices via power banks but you cannot charge a laptop with a power bank.
One advantage of Windows is that you can install full MS office on it. Pay attention that some laptops have Windows RT. They usually have MS Office preinstalled, which might need a license, but you cannot install .exe files.
I tried to read papers on my touch laptop for a year. At the end I prefer to read short articles printed on paper. I only read books and long articles on my device.
I think you should first consider which software you need, then decide which device you should get. With regards to e-book readers such as Kindle,forget them. You cannot install apps or software on them.