Why can I not type into my PDF?
Because it probably has no fields. A form that was printed, filled by hand and scanned — or exported flat — is a picture of a form: there is nothing to type into, and no tool can fill a field that does not exist. This one tells you that instead of failing quietly. The editor on this site places text anywhere on a page, which is how a scanned form gets filled.
Should I lock the answers or keep it fillable?
Lock them if you are sending the form to somebody: the answers become part of the page, so software that ignores form data cannot show your recipient an empty form. Keep it fillable if somebody else still has to complete their part, and accept that anybody can change what you wrote.
Is my filled form uploaded?
No. The form is read, filled, saved and checked in this browser tab. Open your network panel while you use it, or disconnect from the internet once the page has loaded and watch it keep working — which matters more here than almost anywhere else on this site, because a filled form is usually the most personal document somebody owns.
Can it sign the form as well?
It can carry a drawn signature if you add one with the editor first, and that is a picture of your name — which is what most forms ask for. It is not a cryptographic signature and this site does not offer one: that needs an identity authority and an audit trail, neither of which can exist on a page that talks to nobody.
What happens to fields I leave blank?
They stay blank. Nothing is guessed, nothing is prefilled from anything you typed elsewhere, and a field the form marks read-only is shown but not written to even if an answer reaches it.