Sometimes you may want to send a file to someone who doesn't have the program you used to create it... and when this is the case, you will need to convert your file to a format they can open and read.
A pdf file can be opened and read by anyone who has Acrobat Reader installed on their PC - and even if they don't have it installed, they can always download it for free.
The most expensive way of producing pdf files is to buy Acrobat Professional, but if you don't want to pay several hundred euros, you will be pleased to know that you can download a pdf printer/converter progam for free (and if you use Word 2007 you will find it can convert docx files to pdf files).
I'm not going to recommend a program because there are hundreds of them out there and I don't know whether one program is any better than the other, just stick pdf converter download into Google or whichever search engine you use and press 'Search'.
You may find you have to pay a few euros to get a decent pdf converter, but you can probably find a good one for less than €30.