There are many different ways to create a PDF for upload, depending on the operating system and the word processor you are using.
First of all, identify the operating system and the word processing software you use, and which versions you have. For example, you might be using Windows 8.1 and Microsoft Office Word 2007.
If you're using a recent version of Microsoft Word then you can copy and paste the first three chapters of your manuscript into a new document and click Save As. Then, at the bottom of the pop up box that appears where it says 'Save As Type', you should find an option for PDF. This document can be uploaded as the preview.
The most recent version of Word allows you to create a PDF by clicking 'File', 'Export', then 'Create PDF/XPS Document'. Depending on your computer's settings, this will open a new file as a PDF, or simply save the new file to your documents folder.
If you're using different software to that detailed above, just enter the names of your operating system, your word processor, and the task you want them to do, into your preferred search engine. E.g. "macbook pages convert to PDF". There should be several results with instructions on what to do next.
It's also possible to download a free PDF converter, or use one online. Search for "free doc to pdf converter" on Google to browse results - but always remember to read T&Cs carefully when downloading or uploading documents online!