Receive or send and attachment?
To send an attachment, you go to the app with the file in it, select or open the file, and then tap the share icon (looks like a box with an arrow coming out of it). If you have and email option (most do) you choose that. You can email PDF files from iBooks.
If you are receiving your Yahoo mail in the Mail app, then the attachment will either display in the body (if it's short enough), or as an icon with a line around it.
If it shows up in the body, just tap and hold on the PDF. A popup menu will appear with the usual Open In options.
If it shows up as an icon with a dotted gray line surrounding it, tap on the icon to download it first. If the line is already solid (downloaded) then tap and hold on the icon for the popup menu.
It's been a while since I played with Yahoo Mail in Safari or using the Yahoo Mail app, so I don't remember if downloads are possible. Experiment by tapping, tapping and holding, and keep an eye out for the share icon. If it's possible those are the most likely ways it will show up. A menu item that says 'download attachment' might also provide valuable clues.
