If you are browsing PPRuNe on the iPad, there is an option in the Safari browser to select private mode. This clears cookies when you have finished browsing a site and close the tab. That is why you keep coming back to find the mobile site format selected by default.
Using Safari without the privacy settings enabled will preserve the PPRuNe viewing format you select, or stop the BBC and other sites constantly starting with cookie notices, etc. The disadvantage is that so long as your browser history and cookies are preserved, advertisers can track your internet usage.
So either click at the bottom of the first screen to display the full site, or risk being bombarded everywhere you go with directed advertising. The choice is yours.
The iPad file system is pretty locked down when it comes to transferring data into the iPad. Apple would prefer all transfers to only take place using iTunes, but there are third party PC apps that will allow you to read and write files to the iPad. Whether or not native iPad apps will recognise them or run them is another matter. The files generally need to be copied to specific directories that are normally hidden from users when using the official transfer methods.
iTunes will happily play .mp3 files. No need to convert to a proprietory Apple compatible format. VLC is also available for the iPad from the App Store. This app will play most formats without any conversion but lacks some of the convenience and polished presentation of the native music app.
The iPad cannot directly connect to external memory devices by deliberate design. Apple's philosophy is that if you need that capability, buy a Macbook. You can use the iPad as a kind of USB memory stick to transfer files, but these files are firewalled from the iPad operating system and apps. Cheaper and easier to just use a USB stick in the first place.
From an anti virus security point of view, this is a good thing, but it is a very different method of working if you are used to the click and drag technique on a PC.
The iPad is a great device when used within its limitations. It has been deliberately hamstrung in order to avoid being seen as a direct competitor or rival to the Macbook series of computers.