Hey Charlotte.
"Save as PDF" is built into most printer menu options these days. See my graphic below of the printer menu that pops up on my computer. Yours will look different depending on your brand of printer, but they all have the same "save as pdf" option (these days).
(1) Select "printer friendly format" from the options menu of any post. This opens up a new browser window for just that post (and also strips out a few things on the page you don't need to print).
(2) Select PRINT from your browser's menu. This will open up your computer's printer menu.
(3) Select SAVE AS PDF as your printer type. This will show you a 'save' dialog to let you save the pdf wherever you want to.
Ta-Da! PDF 
Helpful graphics:
Below is what my printer's pop-up menu looks like when I click "print" in my browser. I just need to change the print destination to "save as pdf" and it saves it in that format on my computer.
