The most likely culprit is the name of the file itself: refrain from using special characters (underscore is fine) and long file names. With PowerPoint, make sure to save files as PPTX, not the older PPT. With PDF, if the problem is still not resolved then the final solution is to "reprint" the PDF, which can be done by going to File > Print then for printer selection choose Microsoft Print to PDF—note, this action will break any hyperlinks and fillable inputs, but that is more of a concern using the PDF outside of Adobe Connect.
Adobe Connect uses a plugin that while not required to have as a Participant, it enhances the viewing experience and is recommended to install. To ensure your computer will work with Connect and to install/update that plugin click here.
Note: Faculty are required to download this plugin to login as Host. And students, that are expected to upload documents as Presenters, will also be required. Connect does prompt required users in-session to download this plugin if they do not have already.