Bookends 4‏+

Bookends is the reference management app you've been looking for.

The basic version offers:

• Search online sites directly (arXiv, Google Books, Google Scholar, JSTOR, the Library of Congress, PubMed, PubMed Central, and Semantic Scholar) or use the built-in browser (EZProxy-compliant) to download reference metadata and PDFs (access privileges required). Searches can return a maximum of 20 references, and only 50 references can be stored in your personal database.
• Use unique identifiers such as DOI, PMID, ISBN, or arXiv id to autofill references.
• View the text of PDF annotations in a note stream, and create your own with our powerful PDF markup engine.
• Barcode scanning, one per day.
• And many more features designed to help you discover, organize, and use reference information.

Standard, one-time purchase ($14.99):

• No limit on searches or importing.

Premium, yearly subscription ($14.99). Requires a Standard purchase.

• Cloud sync with all your iOS devices and Bookends for Mac (separate purchase).
• Share online libraries with colleagues.
• Scan documents with Mellel Mobile*.
• Create multiple libraries.
• Unlimited barcode scanning.
• Extract PDF annotations with deep links to annotations and text.
• Convenient UI to add PDF notes to the library.
• Automate your actions with Shortcuts.

* iPad-only. Requires a Mellel Mobile Premium subscription.

Subscription is for one year and has an auto-renew option. The subscription can be managed in iOS -> Settings > [your name] > Subscriptions. A subscription will renew if it isn't canceled at least 24 hours before the renewal date.
If the subscription is not renewed, Premium will be disabled but Standard will continue to work.

Terms of Service: https://www.apple.com/legal/internet-services/itunes/dev/stdeula/