This is obviously not from Apple. Always look for typos and unusual requests. This one wants me to save the download to my computer... not going to happen.
Apple Store
Recently, there's been activity in your Apple account that seems unusual compared to your normal account activities.
Read your secure message by opening the attacment.
or
Click here
You will be prompted to open (view) the file or save (download) it to your computer. for best results, save the file first, then open it in a Web browser
Thanks
The Apple Online Services Team
Apple-Invoice( Appstore ) .pdf
Definitely not written by a professional, but good enough to hook most.