Which mission did you download?
Yes for sure. Go to Armaholic and grab this tool:
In your Arma 3 folder in Documents, inside your username folder, make a new folder inside "Missions".
If this is my Inshallah mission, name the folder Inshallah.Takistan or if it's the Tanoa Showcase, name it something like ALiVE_Showcase.Tanoa
Place the PBO in there.
Open PBO Manager. Find the folder path to the new folder you created. Right click on the mission PBO, scroll down to where it says "PBO Manager" in the right click menu options, and select the option to unpack the PBO to the current folder path.
You can now delete the PBO or remove it from the folder if you want. All the mission files will be inside the mission folder.
Open the game and the mission should now be available in the editor for the map the mission is on.
If you edit anything and want to pack the mission back into a PBO. Open PBO Manager, find the folder path for the mission, right click on the entire mission folder, scroll down to PBO Manager in the list, and pack the folder into a PBO.
Then place that PBO in your normal Arma 3 root folder MP Missions folder (steam\steamapps\common\arma 3\MP Missions), and now the mission can be played like you would any MP Mission.
I'm writing these directions on the fly. Not at my PC. If something isn't clear let me know and I'll PM you better instructions when I get home. Or I can send you the unpacked missions directly if you want.