No, it is not possible to access PDF hyperlinks offline in the ArcGIS Field Maps mobile app. Hyperlinks require an active internet connection to view online files or websites. However, PDF files can be accessed by configuring the map for offline use in the Field Maps Designer web app. Adding the attachment to the feature layer in ArcGIS Online Map Viewer before downloading the map area in the ArcGIS Field Maps mobile app enables offline access to the attached file.
The ArcGIS Field Maps mobile app image below shows the PDF file attached to the feature layer for offline use.