I think you'll want to go with the Chaos Scar series. This is a DDI set of adventures for low level characters that should get you through levels 1-5. It's not a campaign per se, more of a sandbox, but should be no problem to string together if you're an experienced DM. I ran some of it with my newbie group, before that group ended, I had planned to run all of it.
I started with the D&D Encounters Keep on the Borderlands adventure; however, the adventure as written is just combat encounter after combat encounter, so I stretched it out and chopped it up a bit. You may have to find it on ebay if you want a print copy, I ran mine from a PDF.
Interspersed in that adventure I ran other side adventures from the Chaos Scar series, of which there are 15 for levels 1-4, which you can find here (DDI Subscription Required): Chaos Scar
1A Stick in the Mud
1B Den of the Slave-Takers
1C The Brothers Gray
1D Death in the Pincers
1E The Tainted Spiral
1F The Lost Library
1G A Chance Encounter
1H Eyes in the Forest
1I Elves of the Valley
2A Sliver's Call
2B The Shrine of Glass-Spire Forest
2C Dead by Dawn
2D The Hammer Falls
3A The Crawling Fane
3B The Splintered Spring
4A Glowstone Caverns
The characters were based out of Restwell Keep, which is detailed here (DDI Subscription Required)
The map of the Chaos Scar is great and can be found here (Free, as a .jpg)
There are two good resources for Chaos Scar info, both D&D Groups are old and outdated, but good info is here and here (Free)
I really liked the background of all the adventures that I ran and those that I read. I was hoping to use more of them but we stopped after 2 levels unfortunately. If I ever run another newb group I would probably use this series again.
After what's listed in the first link there are the following adventures:
LVL Dungeon# Name
- 4 178 The Crawling Fane Chaos Scar
- 4 180 The Pillar Of Eyes Chaos Scar
- 4 182 Vanguard Tower Chaos Scar
- 5 181 The Slaver's Stone Chaos Scar
- 5-7 192 Scarred for Life Chaos Scar
- 6 183 The Radiant Morn Chaos Scar
- 6-8 193 Rumble in the Valley Chaos Scar
- 7 184 Head in the Clouds Chaos Scar
- 7 196 Reflections of Ruin Chaos Scar
- 7-9 189 Scarblade Chaos Scar
- 8-10 186 The Runecutter's Ruin Chaos Scar
- 8-10 190 Pit of Delirium Chaos Scar
- 8-10 197 Heart of the Scar Chaos Scar
I hope this helps!
You have two main places that will have something like what you're looking for.
On the Paizo store, there is an adventure finder, go into e.g. 1st level adventures and sort by "Price, low to high." You'll basically see the two free RPG day adventures and the three PFS starter adventures. Real professional adventures don't come free.
On RPGNow, you can search for free Pathfinder adventures. You'll get more, though for some alternate definition of "professional" - not to knock these various authors, but it's not the same as Paizo quality.
I'm afraid that other than that, even for PDFs, financial outlay is required. You could certainly read through some via bookstore/Half Price Books, loaner from a friend, or even a library (some libraries have RPGs).