There should be no problem sending a .doc or .docx file as an attachment with Yahoo!, but the folks on the receiving end may have something stopping it from getting there or making it viewable. Some places block or quarantine email attachments from ever getting to the recipient since sometime malicious software is transmitted that way - security you know...
Send it you yourself or to a friend and see what it looks like. Figure out what the problem is and, fix it. Find out what the receiver that is complaining is using and try it yourself.
If you are sending it to a recruiter, they may not be using Yahoo! for their professional email, so send it to somebody that uses Yahoo!, Outlook Express, Office 200x, etc. and make sure it looks good in all situations.
I also keep a plain text only copy of my resume for places that will not accept attachments at all.
Open the document in Word, Select All, Copy, open a new text file with Notepad or Wordpad, paste - then you have some hand editing to do to make the text file look readable it is not as exciting as your Word document, but when you are done, you can use it when you need to for those places that say paste your plain text resume here.
You don't want to just open your Word document, copy and paste the raw text into their text receptacle - it look terrible on their end and does not make a good impression. Keep a text copy that looks good handy for pasting when you need it.
This is good for things like cover letters where you have to type one it - keep a generic text copy hand and just paste it in, edit, etc.
Jose