SQL Developer

DIS is the file extension used when a workbook has been saved to the Local Drive using Discoverer Desktop.  It is strongly recommended that you do not use Desktop.  Oracle's stated direction has been toward the Web Editions (aka Plus and Viewer).  Discoverer Desktop is little changed from the 4.1 release.
Each "report" is stored as a Worksheet within a workbook.  I believe you are asking about multiple worksheets.  The answer is yes.  There would be a tab in a workbook for each worksheet (report) that is contained in that worksheet.

It is not clear what they meant by multiple queries.  If they meant multiple worksheets, then you should see multiple tabs. 
I suggest you verify you have opened the correct workbook.  If the workbook was saved locally and also in the Database, both can have the same exact name.

The Discoverer Plus user's guide is located here:  https://docs.oracle.com/cd/E28280_01/bi.1111/b40105/toc.htm

Regards,

Sharon

Please note:  premier support for Discoverer 11g ended June 2014.  Version 11.1.1.7.0 is the final / terminal release of the Discoverer product.  Discoverer is under sustaining support, which limits our ability to assist you.  Please review the alert section at the bottom of this update.

Discoverer is at end of life.  Any software update (OS, browser, Java, WLS, EBS, etc.) released after June 2014 has not specifically been certified or tested and may cause issues for Discoverer.  Therefore, it is important to maintain a stable Discoverer environment for as long as it takes to move to OBIEE or other replacement.

--- Notes of interest

Note 1634827.1 - Oracle Business Intelligence Discoverer Statement of Direction - March 2014

Note 233047.1  - How To Find Oracle BI Discoverer 10g and 11g Certification Information

Note 2129633.1 - Upgrade of Software That Impacts 11.1.1.7 Discoverer Environment

sbeck-Oracle - excellent info, thank you! I will check out the resources.

mrigg - fantastic, thank you. Just what I was looking for.

FYI - I'm on Desktop Client 11.1.1.3.

What do you recommend for a replacement? Is there any tool that could convert Discoverer workbooks?

As Discoverer workbooks are stored in Binary, converting them and the EUL content they use (tables/fields) is not simple.  There may be 3rd party products, but there is none that I know about.  Oracle had created a utility, but it could not migrate/convert everything (from Discoverer to Oracle BI Enterprise Edition OBIEE).  If you use an Apps Mode (aka connected to E-Business Suite) EUL the utility could not be used.  As it is not an Oracle product, it is not supported by Oracle Product Support.  In addition, it only works from Discoverer 10g to OBIEE 10g.  The latest version of OIBEE is 12.2..
The Oracle replacement for Discoverer can be OBIEE or BI Publisher.  You should contact your Oracle Salesperson to discuss your company's needs.

OBIEE has admin software that is similar to Discoverer's admin, that allows you to build you Subject Areas (similar to Business Areas) by importing their definitions directly from the Database tables.

Depending on your needs, there is also OBIA (Oracle BI Applications) which provides pre-seeded OBIEE content.  OBIA will load a Data Warehouse from you Source system(s).

It is best to contact your Oracle Salesperson.

Best Regards,
Sharon.