How to flatten without losing image quality

Hi all!

I am returning to the Adobe community after many years.  One of the reasons I haven't used it since school is that, while I found the individual programs very useful, I've always been overwhelmed by the types and variety of Adobe products!

Here's what I'm trying to do:

- I make files on Google Slides for educational products to sell online.

- Previously, I have downloaded them as PDF files and distributed them, no problem.

- Through research, I have found that the preferred method is to "flatten" a PDF so that images and text cannot be copied from a file.

- I do understand that pieces of my PDF would be available through programs such as a "snipping" tool. 

Here's what I've done:

- I found instructions online how to adjust security settings so that the document's text and images would be unavailable to copy.

- In my normal, factory-installed Acrobat Reader, those options are static - it's a list, not an editable on/off field.

- I found instructions on how to turn a pdf to a series of image files, then back into a pdf.  I also found how to make that pdf more than 12 pages.

Here's the problem with that - the quality of my product goes down!  It goes from a crisp, clean pdf, to looking, well - like a picture of a pdf!  It's not awful, but it's nowhere near what I am willing to sell.

So... I paid for Pro DC.  I cannot install it (since I use my school's computer and don't have admin rights) - but it says I can use it through a browser.  Each time I try, it says I'm logged in... but then tries to get me to buy Pro DC!

I understand there's a simpler way - to adjust the security settings - however, I can't figure out how to open my file in Pro DC so that I can adjust the security settings.  I hope that once this is figured out, my documents will be secure but also maintain a good quality!

I'm a lot to take on!  But... can you help?

Thanks so much!!

Karen