attaching documents to Apple Mail

I see two things that are a problem overall and one that might explain your issue with attaching files. CleanMyMac is a known troublemaker. It can clean your computer right into a paperweight. It isn't that the program is malware but just that it does things that are potentially dangerous. I also see that you have some kind of Symantec anti-malware. This is really bad news. Go to the CleanMyMac and Symantec Web sites and find out how to properly uninstall them. MalwareBytes is the only safe anti-malware program, in my opinion. Now see if you can attach files. If not, read on:

As the report indicates, you have a lot of System Modifiers. Honestly, I don't think I've ever seen so many listed in an EtreCheck report before. If the problem still exists I suggest that you reboot into Safe Mode (hold shift while you boot up until the Apple Icon appears). This will be a longer boot than normal because there are some diagnostics going on in the background. The purpose of this safe boot is that all those extensions won't be loaded. Now launch Mail and see if you can attach a file. If you can, there is at least 1 system modifier that is causing the problem. Other than the two I've mentioned, I honestly don't know where to start.

I see yet a third anti-virus troublemaker, or at least a component thereof:

[Running] com.intego.BackupAssistant.daemon.plist (? f4eb85c2 - installed 2014-08-29)

I'm seeing a number of things like the above that are older than your computer. MacFuse is dated 2008 and I think it has been obsoleted by a new product from the same company. This stuff probably got moved from an older Mac by Migration Assistant. Use the EtreCheck report to check dates on apps and plug-ins. A good rule is: "If you use it, update it; if you don't, dump it."

Have you booted into Safe mode (hold shift after restarting) and tried sending a pdf to Mail? Since a safe boot only loads a limited number of drivers and extensions, if you can pin a PDF we’ll know it’s something interfering. If it doesn’t work in safe mode it probably means there’s a low level issue that reinstalling the OS might fix.

How are you trying to attach to the email?

It might help us to know what software & utilities you are running. There's a diagnostic tool called EtreCheck at the AppleStore (free download) which creates a report you can paste here. After running it, click on the icon I've circled below and paste the report into the window that pops up.