With modern mapping software such as Caltopo, it’s easier than ever to procure or make a PDF file of a good quality map. While you should of course have a printed map with you, why not also have a copy of the PDF on your phone?
(In addition to a map, you can take photos or scans of guidebook pages, or even an entire guidebook as a PDF file, if it's available. Or, you can really go for it and maybe purchase an entire guide book in Kindle format, and keep that on your phone as well.)
The map is free, a PDF file does not “weigh” anything, and it takes just a minute to add it to your phone. Having a backup copy of the map on your phone can be handy if your paper map is lost or destroyed, and it lets you share it with someone on your hike or climb who perhaps did not bring one. You can pan and zoom, and with a good pdf, be able to see even fine map detail.
Android people, I'm not sure about you, but there are several ways to view a PDF file on your iPhone. Here’s how I do it.
Save or download the map to your hard drive.
Email it to yourself and open the email on your phone with the native iPhone email app (not Gmail, Outlook, etc).
Tap the email to download the PDF, this should take a just couple of seconds.
Do a long touch on the PDF file. A window should open that gives different way to view or save it. I use the native iPhone “iBooks) app.
Tap “Open in iBooks”, and the pdf should open for viewing in iBooks.