You should not expect DropBox behavior from iCloud.
Once you sign-in to your iCloud.com account from the PC via the browser, the iWork icon concentrates on Keynote (Presentation), Pages (Word Processing), and Numbers (Spreadsheet) categories.
You can upload respective native files from the PC counterpart documents that the Apple applications can open. Therefore, Powerpoint files, MS Word files, text files, and Excel files fit that critieria. Unfortunately, PDF do not.
To upload any of the above files, you need to click on the categorical iWork tab. So, if you want to upload a MS Word file, you click on the Pages tab, and then the gear symbol to the right, which offers the upload capability. Then you use the Windows Explorer to select the compatible document and a progress bar appears until upload is complete. You would then observe the uploaded document appear among the existing files in iCloud (iWork).