There is something genuinely odd about uploading a confidential file to a website in order to make it confidential. The whole point of the exercise is that the document shouldn't be seen by third parties, and the first thing every online tool does is take a copy.
Here the encryption happens in your browser with AES-256, the strongest cipher the PDF specification supports. Neither the document nor the password is transmitted, because there's nowhere to transmit them to.
You can set the password needed to open the document, and separately restrict what someone can do once it's open — printing, copying text, or making further changes. It's worth knowing that those permission restrictions are advisory in the PDF format: compliant readers honour them, but they aren't enforced by encryption and plenty of software ignores them entirely.
Type into real form fields, or anywhere on a flat form.
Draw, type, or upload a signature and place it on any page.
Number the pages, with control over position and format.
Add DRAFT, CONFIDENTIAL or a logo across the pages.
Cut away white margins or crop pages down visually.
Combine files into one PDF and set the order before you merge.
Break a document into separate files by range or interval.
Drag pages into a new order, rotate or drop the ones you don't need.
Turn sideways or upside-down pages the right way up.
Pick pages to drop, visually or by range.
Keep only the pages you want as a separate file.
Combine JPG or PNG images into one PDF, with page size control.
Render pages to images at the resolution you choose.
Shrink a PDF with a side-by-side quality preview and a size target.
Add a searchable text layer to a scanned document.
Extract text as plain text or Markdown, in reading order.