Access print/PDF using the three dots on the Stories page, under "More" on the View Story page or from "Print Preview" at the bottom of the Edit Story page.

On that print manager page, take a minute to format your story - add a background, a border, if you have a title in your story removed the title.

Then click/tap "Print" to either print or save a PDF of your story by selecting 'Save as PDF' as the destination in your printer dropdown.

Note, in the browser printer popup, under settings, you can also generally manage how the story fits on a page - by changing the margin settings, and by excluding headers and footers (such as the story URL that some browser include by default, but which can be removed).

Print/Save PDF design and layout options - more here