technical knowledge all fall down

I have this PDF that is on our website that I need to edit, fairly substantially. I don't have access to the original document, as the person that did it in the first place is away til mid January.

I have very little understanding of PDFs - as far as I can tell from my googling, to change it I'll need to convert it to word, then re-convert it back to a PDF?

I've tried a few free downloads of PDF editing programmes, but they've all been rubbish, and the only free converter I could find would only convert 5 pages unless i paid for it, and this is 8 pages long.

Help? Can anyone explain to me in small and easy to understand words how I do this? Anyone know of a decent free programme I can do this with?