numbers
I often find it useful to have two tabs of one spreadsheet open in separate windows. In Excel this is easy (Windows/New Window).
Is there any way to achieve this in Numbers (as of Version 3.5)?
For the record, I've tried searching for an answer to this, but needless to say, searches including "Numbers" and "Windows" doesn't lead to anything useful.
This screen shot is from Keynote for iPad, but Numbers for iPad is pretty much the same:
When you view the file in Preview mode, the title bar of the document should look pretty similar to the above. Over there on the right is the Open in… button.
There might be a 
button that you have to click first before you see the Open in… button.
Other ways to transfer files to your iPad:
I had the same problem. A hard reset of my iPad (press and hold both the home button and the power button until the device turns off, then wait while it reboots) solved the issue.
Best Answer
Thinking about this some more, I have figured out a workaround. Though it's not altogether satisfactory, it could be useful in some cases. The workaround is iCloud.
Save the Numbers spreadsheet to iCloud drive, and then open it from a web browser. Open a new browser window and again open the spreadsheet from iCloud drive and you now have the spreadsheet open in 2 windows.
You could also have a local copy open in Numbers, with additional windows provided by iCloud browser windows, though there would be some lag between local changes appearing in iCloud and visa versa.