For quite a long time I thought you couldn’t combine your stray PDF files into one, but apparently it can be done. What’s more, it’s straightforward, requires no downloads (aside from your new PDF file), and it takes only a couple seconds. A quick Google search brought me to PDFMerge! in particular, and it worked so well and so easily I haven’t even explored the alternatives yet.
Looks like you can also split PDFs in case you realized you merged / scanned / converted / etc. things out of order. The only limitations I’ve seen is the size limit of 15 mb, but that’s well within the range of most PDFs people will be using.