Why are there automatically generated text boxes on top of the already existing text in my pdfs files?


I do not know why, but with some pdf files, Libre Office generates small text boxes everywhere on the document that makes it very blurry (since it does not fit the text already existing). How can I massively get rid of these boxes? There must be a more efficient way than deleting them one by one…
Can I prevent the automatically generated boxes to happen on other documents?

Thanks in advance