Android and iOS devices have PDF converters that you may not even know about. In the most simple case, for an Android device you should be able to create a PDF from an open email message or web page by tapping the three dot More menu in the upper right hand corner. When you select a printer you can choose to save it as a PDF.
With iOS 10, when you have a page open, tap the Share menu icon and select Print. Put you fingers on the screen and pinch them outward to create a PDF of the document. Tap the Share menu icon again to send or store the file.