1. Open the file of interest in Safari
2. Tap Safari's address bar (the keyboard will appear)
3. Go to the very beginning of the file's URL address, where the http:// or https:// part is
4. Add a single letter g in front of the address, so you'll get ghttp:// or ghttps:// instead of http:// or https://
5. Press Go on the keyboard (this will close Safari and start GoodReader)
the file will start to download into GoodReader automatically (check Web Downloads section to find it)
Note: If you plan to do this often, you can simplify this process substantially. In GoodReader's settings, General section, tap the button called Put special address to clipboard. Then go to Safari and create a new bookmark. Bookmarking absolutely any page will do. Name this bookmark, for example, Save to GoodReader. Then press the bookmark button, and then Edit. Select this new bookmark. Now Safari will let you modify an address of this bookmark. Delete the old address and paste the text from the clipboard (GoodReader has prepared this special text for you). Save the modified bookmark address. Selecting this bookmark when a page of interest is opened is the same as adding the g letter in front of this page's address.