Receiving PDF attachments in html format.

PDF Attachment Mails received are in HTML format and I uanble to open it. How to receive it in  the same format as sent by the sender

You can change which format it is displayed in by going to Menu > Tools > Settings > Mail > Read and untick Read all messages in plain text. That will display the message as it was sent instead of forcing plain text. So if the message was sent as HTML, then it will be displayed as HTML. And if they sent it as plain text, then it should be displayed as plain text. 

But whatever the format of the message, you can open the attachment by clicking on it. Or you can save the attachment to your computer and try open it that way. That is the same regardless of which format the message was sent in, or how it is displayed in eM Client.