Is it safe from hackers if I save private keys in a pdf or txt file and store it in a USB flash driver. This USB flash driver will never touch a computer since then?
Safe when it is not connected to a computer. However, flash drives can get corrupted, so it is better to have at least 2 flash disks.
For added security it is smart to NOT name your file, super_secret_keys.pdf or private_keys.pdf or Bitcoins.pdf or something like that.
But rather name it something that a hacker wouldn't be interested in reading. Examples: "Libreoffice.3.1_manual.pdf" or "Introduction to discrete Maths.pdf". Fo r fun the beginning of the document could actually be just that. Maths or something fun.
And do not ever mention words like "private key", or "bitcoin" inside the document because those kind of words can be automatically searched from files on the disk...
You could also make the pdf password protected.
Also good to have lots of files on those USB:s to mix things up a little.
Is this too paranoid?