There are many PDF readers to choose from:
Others can be found by googling "PDF reader", some are available as free download, some may be free to use while online, others may not be free. Some may also have ability to read ebooks as well as PDF.
Many internet browsers, including: Firefox, Google Chrome, Opera and Microsoft Edge, can open and display PDF documents for you, so try opening any PDF file with your internet browser; if this works for you, then you do not need a specific PDF reader.
If you use your browser as your PDF reader it is likely that your browser's colour scheme and fonts will override the colour scheme and fonts of the PDF document. This may be an advantage if you prefer your browser's colour scheme and fonts.