Open-Source Alternative to Adobe Acrobat

Hi all. I have been using adobe acrobat for as long as i can remember, the main features i use and like are:

  1. PDF Viewer

  2. Form Filling/Editing - Even if there isn't a form, acrobat let's us create a form, add/remove fields and boxes, whatever we want

  3. Saving/converting the file as a word document or jpeg images etc.

  4. Combining many images in a folder into a pdf etc.

Problem with acrobat is that its comes with a whole lot of other unnecessary crap that run in the background even when you aren't using acrobat, they eat up ram, cpu and slow down the system overall considerably. So I was wondering if there is an open source software that'd let me do all of the above i mentioned.