The attachments I send to Windows users can't be read
To make files you attach to all outgoing messages "Windows-friendly," choose Edit > Attachments > Always Send Windows Friendly Attachments. To attach a Windows-friendly file to a single message, click Send Windows Friendly Attachments in the Attach dialog for that message.
If you send attachments to Mac users with the "Windows-friendly" option selected, your attachment may be unusable or may be missing information (such as the preview icon for the file).
Use filename extensions, such as .doc, in the names of files you send to Windows recipients. This way the files can be read by a Windows application.
Certain links that this article refers to may not be available in this context; please consult the relevant product Help guide page on your computer for full access to these links.
Ensure that PDF files are created using distiller
All PDF files should be created using Distiller as opposed to the export function of the graphic edit software. Often a PC will not be able to read the PDF file created on a MAC that has been created without Distiller