Selecting something when displaying a PDF file in a PDF viewer has nothing to do with LO Writer.
Remember that PDF is a page description language and, indeed, you get images of pages.
But a PDF viewer can make a smart use of this description language provided you ask it to do so. My default PDF viewer on my system is Okular with a Tools
menu. By default , the “navigate” tool is selected. I can choose “text selection”. The mouse cursor then changes in a caret-like one and I can drag this tool over text to select a range.
Note that PDF is akin to some graphic scene and a viewer cannot make any difference between line wrap and paragraph boundary. Consequently when you paste copied text from PDF, you get a sequence of paragraphs and you must remove paragraph marks in order to restore original paragraphs.
The answer to your question is: no need to enable some exotic option in Writer; just select the adequate tool in your PDF viewer.
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In case you need clarification, edit your question (not an answer) or comment the relevant answer.
(Nothing new in the edit: I just removed a typo)