A virtual PDF printer allows you to create PDF files from any other format. It is possible to convert a file into PDF simply by opening it with the relative program (wordprocessor, spreadsheet, text editor ..) and printing it as you would normally print it, except that instead of selecting a hardware printer you select the virtual PDF printer. This effectively creates a PDF version of the file that we printed via the virtual PDF printer. With this procedure, you can convert virtually any type of file to PDF format.
The free Adobe Acrobat Reader is a PDF viewer and does not allow to modify PDF files. Rotating a PDF and save changes would require to save a modified version of the PDF file, this is a premium feature available in Acrobat DC.
PDFsam Basic is the most popular offline PDF merger. No need to upload or share your PDF documents, keep them safe on your computer, away from prying eyes and easily merge them with PDFsam Basic. Completely free and open source, without time limits, without watermarks, simply free.