File Sharing, No Account

My apology ahead of time; I'm sure this question has been asked many times in many forms.

I have a Dropbox account and I've uploaded a PDF to Dropbox to share with people in general on social media so I don't bother them with long Instant Messages, downloads, etc.

I've assumed recepients can click on the Dropbox link I provide in an Instant Message and the PDF will open up WITHOUT first my account page appearing where the file/folder resides (and then another click to open the PDF).

One recepient advised she tried to open the PDF file but indicated she has more than one Dropbox account (??). (Or that she simply could not access the file without first logging in, having forgotten her password, etc.).

My intention is to share the PDF not knowing if the recepient has a Dropbox account. I just want a way for people to click on the link I provide and read the PDF without editing permission.

Upon reading through the Dropbox website, I've come across "file Sharing" without an account, but then I see "Desktop App" and Android App".

In simple terms, for a non-technical person:

1. Do I actually need the Dropbox app to share a document (photos, whatever is supposed) so the file item opens up directly for the recepient?

2. Does the recepient need such an app or a Dropbox account?

3. Or, can I simply copy-paste the URL of the document (photos files, etc.) and send that as a link (as I've been doing) so the recepient clicks once and it opens?

4. Or, would I have to add something to the URL copied link for the document to open up automatically for the recepient?

5. When a Dropbox account is created, is the default setting Public? (I believe I've hidden a few unrelated files, but one never knows).

Thank you.