Firstly you need to
read this regarding Calibre and PDFs
Secondly, Calibre will not help you reformat a PDF into another, more readable, PDF.
You don't say what your reader is, but Calibre will convert PDFs to other formats, e.g EPUB (for Sony readers), MOBI (for Kindles). The results vary from "not bad" to "horrible". I would expect a magazine, with images and multi-columns, to be at the "horrible" end of the spectrum.
If you want to read a PDF, originally designed for a large printed page, on an electronic device then you need a reader with a bigger screen, e.g it may be OK on an iPad or other tablet, although I can't speak from experience as I don't have one.