The documentation takes up several thousands of pages, so there's no single PDF.

You can get free PDFs of tutorial collections (or order printed versions for a charge) here. Even these PDFs are typically longer than 100 pages, so it's usually more practical to just use the (hyperlinked and searchable) web version.

I should have mentioned that these PDFs seem to be extracted from the documentation of version 9. This means that the information within is mostly correct for version 10 as well (Mathematica tends to stay pretty compatible), but some things will be missing. In particular, all cloud-specific features (deployment, etc.) will be missing, and there will be no mention of associations (a core language feature which is new in v10).