My aunt got an iPad, and asked me how to download PDF files on it. When I got to her home, she told me that she already had the files in her iPad, but when I took a look at the files, I realized that she created a direct access link to the PDF in the web, but not really downloaded them. So, at first I downloaded the Adobe Reader to found that it only allows users to read the PDF but not actually download them. However, after a quick search I found a really cool tool that was in the Apple Store, for free!! and that allowed me to do what I needed and more!!
The tools name is Readdle Docs. It is a very easy to use tool, and it allows you to download files (it also works with images, videos, Office files and more) without using a computer, but also allows you to highlight the text of the document, add notes, underline text and more.
It is such a great tool that it also allows you to synchronize your documents with different apple products and also from different clouds services such as Dropbox and Google Drive, just to mention some examples.
I strongly recommend this tool if you as me, are looking to download files with no need of a computer.