The email may also contain zip and these files contain executables that are disguised as PDF files as they have a PDF icon and are very named FORM_101513.exe or FORM_101513.pdf.exe since Microsoft does not show extensions by default they look like normal PDF files and people unknowingly open them.
The email may also contain ZIP files and these zip files contain executables that are disguised as PDF files as they have a PDF icon and are typically named something like FORM_101513.exe or FORM_101513.pdf.exe. Since Microsoft does not show extensions by default, they look like normal PDF files and people unknowingly open them.