Slideshare (http://www.slideshare.net/) is a popular place for people to upload documents.
Downloading for many documents does not seem to be enabled but this can be done using the online tool https://www.extramaster.net/tools/downloadFromSlideshare/
For me this downloaded a zipped collection of jpegs. With a bit of file renaming and the
convert application (part of ImageMagick; see http://www.imagemagick.org/script/convert.php for details) it was not difficult to combine these jpegs into a single pdf.