This greatly depends on what exactly it means by "will not open".
The most common reason is that some email providers will encode the pdf in transit to make the transfer faster. Unfortunately, that can corrupt the pdf at the other end. This could be the problem if they are getting a message telling them something to that effect when they try and open the pdf. The fix to this would be either to Zip the file first and have them unzip it at their end or, place it on a document sharing site and give them a link directly to the file.
The latter of course has another set up potential issues since many web browsers use their own pdf viewers that may or may not open pdf files properly. This is especially true in the case of pdf forms. If that happens, they must right-click the link to the pdf file and choose to download it to their computer rather than trying to open in the browser.
If neither of the above issues/solutions are ringing any bells, we will need more detailed information from you on the exact steps being taken and the results.
When the receiver tries to open they get a message that states there is an internal error-- this only happens when I send from my yahoo account on Outlook. I know I can just send from my other accounts but I like to keep them separated.
If it is Yahoo specifically (and works in others) then as mentioned above I suspect it is due to encoding the file which has been the guilty party for corrupting pdf files from way back. Other than the workarounds I mentioned above, there isn't anything to be done.