Using PDFs in a questionnaire may be a bad idea. You never know, if the respondents' browser shows the PDF like in image within the page, in a separate tab, or if they get a download, and must open that separately. Except, you can choose to always privide a download. But even then you don't know what program they use to view the download - some may not even have a PDF viewer installed.
When you have decided what your preference to embed the PDF is, please take a look on this manual from W3C: How to Embed PDF in HTML - in the end, you just have to vary the PDF filename which works exactly like it works for images or videos, just the HTML code around is slightly different.