Welcome to Android Central. To reply to this thread and receive notifications when others reply please register an account. This link will help you do that...
https://forums.androidcentral.com/ask-question/409154-join-android-central-community-new-post.html
You mention Yahoo specifically, is this because you're using the Yahoo email app or because you're getting the PDFs from your Yahoo account?
Have you defaulted to open PDFs with the ES File Explorer app? Have you tried another app for opening and viewing PDFs? If you have defaulted to any app you can remove its default status if you ever want to change by going to Settings>Applications>any app name>Defaults>remove from default.
What device do you have?
What cloud service were you using? Is there a discernable difference between the PDFs you can only view and the ones you're able to save? Source, encryption, size, link, attachment?
Did ES File Explorer come preinstalled on your device? This is kind of an off topic question because I thought ES was being pulled from Google for violations... I just looked it up and the reviews are not good. Just as many 1 star reviews as 5 star.