How to maximize security in PDF (containing no links, no media, only images) from external source?

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How to know which setting to maximize protection when I download PDF from external websites?

These PDFs are product user manuals which have none of the following:

1. clickable link
2. multimedia

I am stumped on how to:

1. avoid unknown sender when Product User Manuals are PDFs?
2. test whether PDF is not executable?

According to following link avoid unknown sender; ensure PDF is not executable 

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Is it possible to export setting related to security? Otherwise, it is tedious to change security setting when there is need to open internal and external PDFs on a daily basis,