CAn you please tell me what kind of device you are using? The behavior you describe is unexpected on any device but it will help to troubleshoot the behavior if we can start with the device type. Each device has a slightly different way of managing and presrnting local and recent files.
You could try to search by typing the file name.
Check where the files were saved.
Try to locate by the path if you remember.
If you have accessed it through Reader,try to check under the Recent List.
I am using Samsung Galaxy Grand Neo
We really appreciate you taking the time and effort to report the issue. We would like one specific piece of information that will enable us to investigate and quickly provide you with a fix.
Would it be possible for you to re-download one such bill (from your email account) to your device and share the folder's path it is getting saved to ? You can find this info in the section header just above the file's entry under "Local" tab inside Adobe Reader. Please do remember to refresh the file-list by swiping downwards once you have downloaded the file.
Here is a screenshot of what we are looking for (note the blue rectangle) :
Adobe Acrobat team