Hi Ria,
This is difficult to answer without reviewing the actual pages in question, but I’ll answer this with my general approach.
I see the approach to PPC and SEO pages as different for the most part. There are instances where it makes sense to use an actual website page as the destination; it just really depends on the product/business.
The PPC pages should be tightly woven with the ad groups and keywords, and sometimes “non-PPC landing pages” are too distracting with everything else on your website considering you are paying for each click.
If you take the above approach, then I would noindex the PPC landing pages as they probably contain content that is duplicated from your website.
Hope this helps. I’m also happy to look at specific examples. It’s very hard to say why organic traffic is down without understanding more about the actual situation.