I find that in
most cases Calibre does just fine at converting epubs to mobi.
A couple books have some formatting quirks, but most are fine.
You might download Calibre and convert a couple and then view the results.
And of course you can always send the kindle back if it doesn't work out.
On PDFs with a DX the multi-column papers might be viewable as-is unless you
have poor eyesight and want bigger fonts.
Or you can try this:
http://code.google.com/p/papercrop/
or
PDFread
https://www.mobileread.com/forums/showthread.php?t=21906
Both claim to convert multi column PDFs to single