Android - PDF reader app that remembers the last page you were on?
Dropbox wasn't made to be a PDF reader, instead it's an application to have your files wherever you go. So I have two great suggestions for you:
ezPDF Reader - This is the best PDF reading application I ever seen for Android. I use it every time I need to read any kind of PDF on my Motorola Droid.
Adobe Acrobat Reader - A free alternative for ezPDF. Made by Adobe of course, which is very good to read your PDFs on-the-go too.
Both will restore from the place that you left
Aldiko is an eBook reader that reads PDFs as well, remember the last page you were at and on top of that allows you to set bookmarks, something that Adobe Reader and most other readers I tried won't let you do.
Bookmarks are a must have for me so I chose Aldiko.
Moon+ Reader will sync pdf files from Google Drive and resume from where you left off across multiple Android devices.