Getting the PDF (legally) is step one. Reading it is step two. Many users download the PDF and give up after 10 pages because they don't know how to use it.
Many Central Government Ayurveda colleges (like IPGT&RA, Jamnagar) have digital repositories. While they may not host the full book, they often host the first few chapters as "preview PDFs" to help students decide if they want to buy. Searching "L Mahadevan Charaka Samhita PDF preview" yields legitimate academic links. L Mahadevan Ayurveda Books Pdf