Every time I open a saved PDF my MacBook Pro converts it to a word document. Why is this and how can I prevent it from doing this?
It sounds to me that your system is set to use the wrong application.
Are you getting this dialogue box?
This indicates that your system is set to open a pdf (or all of them) in Word. This can be changed by clicking (or Command & I) on a pdf, right-clicking and choosing Get Info.
Look for the "Open With" option and see what it says. For a pdf file it ought to say Preview (or at least another pdf reader). If it says anything else such as Microsoft Word then change it to Preview then click on the Change All button below this option.