Digital receipts should be fine.
IRS Publication 583 outlines a complete list of purchases and expenses for which it encourages business owners to maintain records (most importantly mileage and meals). A specific method of record keeping is not required, but you should have enough documentation to verify any expense or purchase you claimed on your tax returns, in addition to verification of all business income. Organize receipts by fiscal year and category for easy future reference.
You can reference IRS Publication 583 here:
https://www.irs.gov/pub/irs-pdf/p583.pdf