PDF Viewer supports opening of PDF files from virtually any location.
Local filesystem: PDF files can be read from the local filesystem including the Download folder, the internal mass storage, and external SD cards. To enable access to local files you need to grant file read permissions when initially starting the app.
Shared from other apps: You can share and open PDF files from within most third-party apps.
External devices: USB-OTG devices (for example external hard drives) are accessible via the "External document" menu.
Document providers: Third-party storage providers (for example Google Drive, Box, OneDrive) are available via the "External document" menu.
The current beta version has a much improved SD-Card & external drives integration. To participate in the beta, follow the steps described here.