If a document was written in MS Word or Libre Office it can easily be converted into PDF: Re-open the file with the corresponding program and klick "Save As..". A window should appear, where you can choose the file format, in this case PDF.

There are also different programs (e.g. WordToPDF) or online solutions for converting these documents. Please note, that not all of them are trustworthy.

To convert .hml-files or webpages into PDF, one can use Adobe Acrobat DC. As before, there are also online solutions for conversions like this, which are not always trustworthy.