With iOS 11, Apple enhanced and improved the builtin Notes app, including a somewhat hidden feature which is easily overlooked but of great use if you ask me: Notes has a document scanner. Apple mentions this feature in a corresponding support article. It works roughly as follows:
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Tap Notes and create a new note (tap the edit button in the lower-right corner).
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Tap the ⊕ (Plus) button, either in the top row of the virtual keyboard or in the bottom row of the screen, and tap Scan Documents.
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Hold your iPhone over the document to scan and either tap the shutter button or a volume button or wait until your iPhone automatically makes a photo.
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Adjust the corners of the scan, then tap Keep Scan.
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Tap Save.
You can edit your scan afterwards, convert it to PDF, share it, and apply filters. What is missing is OCR. If this is a feature you need, several document scanner apps having this are available in the App Store.