Can I do this with Nextcloud?

I want to use Nextcloud to allow clients to download PDFs. I also want them to login.

Can I send a logged in user to a particular folder rather than they go to the standard dashboard?

Is it a strict need that they provide username and password?

If not, the share feature allow you to set a password on a share. So, send the client a password protected link. That will take them to the file/folder immediately after providing the password

if he clicks: he will be redirect to the Documents folder.

i couldn’t find any settings that a user is always to a specific folder.

nevertheless he has read/write access to your nextcloud. if he should be only allowed to download pdf a password protected link would be better.