Make a .tex file that combines complete .tex documents in subdirectories

I'm using LaTeX for my research journal (similar to WordPress).

There are a number of subdirectories that contain complete (standalone) .tex files along with PDF files that I made from those standalone files.

I'd like to get a .tex file that contains all the .tex files and compiles to a PDF. What would be a good way to do that?

Currently I compile the .tex file in each subdirectory and then merge all the PDFs into a main PDF file. The problem is that each PDF takes a page and if I have a small note, the most of the page is clear. Please keep in mind that I would like to keep the standalone PDF files, too.

The other option is to cut out the body of the document and paste it into the main LaTeX file, but i was wondering if there is a package that does it. Meaning that would keep both main and stand alone files. Kind of like have chapters of the book compile separately