References are bookmarks in PDFs and, from AE/AO version 2017.1, worksheets in spreadsheets.
You can link to worksheets with Excel 2013, Excel 2016, and XLS, XLSX and XLSM file types.
In password-protected files, you can only link to the entire document, not to references in the document.
To use bookmarks when linking the document, make sure you've added bookmarks to the document before you start. For example, using PDF Manager, you can save the bank statement and asset purchase contract as one document with bookmarks for each section. You can then link the bookmarks to specific accounts in the Trial Balance (Workpapers). From AE/AO version 2017.1, you can also link to spreadsheet worksheets.