A document arrives with the wrong icon. Renaming its ending seems like a quick repair, but the ending and the contents have different jobs. The extension helps Windows choose an application; it does not rewrite the bytes inside the file.
Show filename extensions in File Explorer. A name such as notes.pdf.txt is still a text file as far as the final extension is concerned. Before editing the name, make a copy and find out which application created the original. An unfamiliar icon alone does not establish corruption.
Open a known, trusted file in an application that understands its format. Use Save As or Export to create the required format, then reopen the result. For example, a plain text note needs a PDF export or print-to-PDF operation, not a renamed suffix.
If someone merely removed the correct extension, restoring it may help the right application recognize the file. Keep that limited repair separate from conversion. A file that still will not open needs identification, not a series of guessed endings.