Hi CarolinaNikki,
It is possible that there is security applied to the PDFs that is different from an open-document password. For example, the person who created the PDF may have set it up to require a password to print or select text or images. You can tell by looking at the title bar of the PDF when you have it open in Reader. Do you see the word "Secured" in the title bar? If not, we have a bigger mystery to solve, so please let us know!
Best,
Sara