Suppose I had a file (a pdf file) on an infected windows 10 PC, I use a usb drive to transfer this file from windows 10 to a Macbook.
Can this Macbook then be infected? Or become a carrier that infect any other usb drives connected to it?
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Depending on the circumstances, nature & capabilities of the virus/malware.
The very fact that a USB drive is writable and does contain memory space makes it susceptible to being infected.
However, infecting a new host with a different OS (in your case, MACOS) is also another set of risks. Certain viruses/malware are created to cater to certain machines and OS hence if they are configured in a way so that they could run on cross-platforms then yes they can infect your new host.
It is also good practice to turn off auto-run/auto-play when USB devices are connected to a machine. This prevents programs from running automatically upon insertion.
Now let's move on to the file(PDF). In the above paragraphs, I have discuss the idea of plugging in the USB drive to extract the said file and the risks of that, but how about the file statuses? Is it infected?
The answer is that any file can be infected & armed by other viruses/malware, to determine whether a file is malicious is not solely based on its file extension.
All that being said above, it boils down to the very nature of the virus, there will be risks & vulnerability involved, for sure. Just 'virus' itself is very vague.