I was wondering if I can use free resources such as freepik.com content to create online banner ads for my clients? Is it legal to do so?
freepik.com lists the copyright information of each resource on it's download page. Most are free to use with attribution, so you need a notice or link to the download when you use it. There is an example of how to attribute next to the copyright notice on the download page.
Wherever you download free resources from, always check for licensing information, there will usually be an FAQ page or notice somewhere. If there isn't, it's probably safer to assume you can't legally use them.
You can find more free resources in this precious Q&A: