While there are so many apps on iPhone to save webpages for offline use, a feature on Safari lets you save and convert a webpage to PDF on iPhone, iPad and iPod devices running the iOS. To convert webpages to PDF on iOS running devices, do as follows:
1. Open the Safari app on your iPhone
2. Browse to the webpage you'd like to save and have a copy in PDF format
3. Tap the Share button from the bottom bar (it's in the middle)
4. Swipe the first raw of action to the Save PDF to iBooks button
5. Tap on it and wait for the PDF file to be created and from now on, you get to access the save webpage in your iBooks library.
Note that, this method will not save the entire website and it just saves a single webpage for offline use.