Since you haven't used ls -le as recommended, we don't know what the access control list looks like, but there might be something going on there that is preventing you from being able to write the files. The fact that a newly created user CAN edit the files makes me even more suspicious.
This command will remove ALL access control list entries from the folder and all its subfolders and files:
sudo chmod -RN path/to/folder/containing/files
Once you've cleared the access control list, try resetting the ownership like so:
sudo chown -R ulrikdamn:staff path/to/folder/containing/files
Now grant permissions like so:
sudo chmod -R u+rwX,g+rX,o+rX path/to/folder/containing/files
This gives you full read/write permissions. The "staff" group and all other users get full read permissions. Using an uppercase X, sets the execute bit for all directories within the tree, but leaves the execute bit for regular files as is.
Lowercase x would set the execute bit for all folders AND files. The execute bit must be set for a directory in order to read its contents. If you'd like to give write permissions to the group or others, switch out for g+rwX or o+rwX as necessary in the last command.
Here I'm passing in a folder path. If you want to pass in a single file, just remove the -R from each command. The R makes the command recursive, applying it to an entire directory tree. Without it, the command will modify the permissions for a single file or folder passed in.
Best Answer
You are right that this isn't a feature of OSX. It looks like it is showing you the code of PDF files instead of rendering them as PDF files. My guess is you've either installed a QuickLook plugin that is doing this, or you've installed an application with a QuickLook plugin that is handling files instead of the default plugins.
There are three folders for QuickLook plugins. Please check these folders for any plugins that you've installed yourself that could be doing this. "/System/Library/QuickLook", "/Library/QuickLook" and "/Users/YOURUSERNAME/Library/QuickLook".
Alternatively you can run the command
qlmanage -m pluginsto see a list of plugins on your system. This will tell you the plugins from these folders, but also includes the plugins that come bundled with applications. You could even filter it to just show those that affect PDF files withqlmanage -m plugins | grep pdf.Let us know how you get on.