the site is looking for a different file type than .pdf, that may be why they are not showing

make sure you are in the right directory where the files are located on your PC. It might be defaulting to your "Documents" folder, instead of the Desktop, or other folder where your .pdf's are actually located.

Do you have Adobe reader? Have you tried looking in your download folder?

You can try opening them again and doing a "save as" to your desktop or a folder on your desktop. Then when you upload, navigate to that folder.

It could be you're clicking on shortcuts to the files when you open them. Or your opening them in your browser and not a regular PDF reader like Adobe.

When you are selecting the file that you are trying to upload there is usually a standard dialog box that shows the files in a folder. Check the bottom edit field. This indicates the file type(s) that it is looking for. You can usually change this edit box to "all files".

The server might only be accepting text files. PDF files are not text files so after after you upload the file be sure to download it to a different folder and open it to see if it works.

To upload files from a device to a website is dependent on a script or an inbuilt website tool. This (whatever it is) may not be configured to accept pdf documents.

Are you able to have a dig around in the innards of the site to find the culprit? A good place to start is the page that contains the upload options.

My habit is to have anything I plan to upload sitting on my desktop, even if it's just a copy of the original file. It's right there on top. I can delete it later if it's a copy, no problem. But it saves me a lot of hunting while I'm in the middle of something.