PDF is a supported type of file when sending receipts to QuickBooks Online. Can you tell us what happens every time you email a PDF copy of the receipt?
Sometimes, the cache stored in your browser gets corrupted and can cause issues with some features in your account. To rule out any browser-related problems, let's try opening your account in a private browser or incognito. This mode prevents your browsing history from being saved.
Here are the keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once done, log back in and email the receipt or bill once again. Then, go back to the Receipts tab and check if you can already see them.