If you install doPDF, you'll be able to create pdf documents from
any program that has a 'Print' option:
https://www.dopdf.com
All you need to do is to go to File > Print (NOT Save) and select doPDF as your (virtual) 'printer'. The program will then create a PDF document that mirrors the content of original file. (i.e. if your original file has 10 pages, you'll get a 10-page PDF document from the process).
However, although it would work perfectly with OpenOffice Writer, you simply don't need it. To obtain a PDF document from OpenOffice writer, simply go to File > Export as PDF. (Once again, the output will mirror the original document, with a 10-page Open Office file being converted to a 10-page PDF).
If you've already got lots of separate PDFs (e.g. as the results of scanning several documents), the simplest way to join them might be to use an online service, such as one of these:
https://www.foxitsoftware.com/merge-pdf/
https://smallpdf.com/merge-pdf
https://pdfjoiner.com/
https://www.sodapdf.com/pdf-merge/
However there are free apps that can do the job too, such as this one from the Microsoft store
https://www.microsoft.com/en-us/p/pdf-merger-splitter-free-pdf-splitter-to-extract-pdf-and-combine-pdf/9p4tcns9h432
or this from the reputable SourceForge collection
https://sourceforge.net/projects/pdfsam/