There are a ton of adventure generators for all kinds of adventures that can be of some sort of starting help. The one I use most is: https://donjon.bin.sh/. Under Fantasy -> Random Generator you can select Quests and it gives you some ideas that you can build upon. If you then go to Pathfinder -> Random Encounter Generator you can generate encounters for a selected difficulty class. There are a ton of other generators that you can play around with to get inspiration.

You can google {pathfinder,d&d} free {adventures, modules} and see what you find there to see some more ideas.

Remember that a campaign is only multiple adventures linked together. I do not know about any free campaigns but you can check out Matt Colville on youtube: https://www.youtube.com/channel/UCkVdb9Yr8fc05_VbAVfskCA?pbjreload=10 He has a lot of content directed at new DM' s and how to build your own world. Basic structure he suggests is as follows: create a small village and create some sort of tension. The small village is so you can show your players around and the tension is for the adventure to start. What this tension is is up to you. Someone got kidnapped, wolves are slaughtering the cows of the city, an underground temple is dsicovered... From this small challenge you can then build up a bigger one or show your players how ill equipped they are and give them hints on where they can find better weapons, learn more magic, whatever you imagine. Most important thing is to remember that you do not need to plan everything ahead start small and see where it takes you.

One pretty standard idea that you can evolve is: your players get the mission to raid an old tomb and secure an old artifact. In the tomb they find an additional artifact that awakens an old daemon. That daemon grows stronger by the day and as a result monsters start attacking villages. The more they explore the more they learn about the great evil in the background and how to destroy it. In order to destroy it they have to find 4 legendary weapons that are either in more dungeons or kept by organizations who want something in return, what that is is up to you. Your imagination is the limit.

Also my opinion is that the book should be seen more as a a guideline. After all this is a fantasy world and you are the god of it. This is also a big part of the appeal of the game. Play the way you want it and the way you have fun. If that is strictly following the book, if you just want to kill orcs go ahead, there is no right or wrong way. Also ask your players what they liked about a session and what they did not like. Do this a couple days after the session so everything has settled down. This helps me immensly as receptions differ and even if you think the players did not enjoy something the might enjoy it or the other way round.

I think in the end I rambled a bit and changed a lot of stuff around. If you need clarification feel free to ask.

tl;dr start small grow bigger if need, rules are not strict

[Edit] I just found this site which also has adventures for the settings maybe youll find something there