It may sound funny to say it, but PDF files (electronic copies of printed documents) are still around and still very useful. PDFs, short for Portable Document Format, allow you to create a digital version of a paper document, making them much easier to store and access than having to search through a filing cabinet.
While there are a variety of PDF apps available (Adobe Acrobat being the most well-known), you don't actually need to download one to create a PDF from an existing paper document using your iPhone or iPad. You can use Apple's Notes app.
Here's how to create a PDF on your iPhone (the process on your iPad is very similar):
If you have existing documents that you want to save as PDFs (in the File app, Safari, Photos, or other apps), you can also do so using Markup or Print, depending on what's available in the app you're using.