You have two problems: 1) using Insert PDF correctly 2) storing multiple PDF files
1) Insert PDF is a FileMaker 12 feature. It only works if the container field into which the PDF is to be inserted has been "optimized for interactive content". Select the container field while in layout mode, select the data tab on your inspector and scroll all the way to the bottom to see this optimization option for your container field.
2) Container fields can store only one file or one reference to a file. To store multiple files thus requires multiple container fields. The best way to do that in your case is to create a table of related records where you have one container field in each record. Each time you want to insert a PDF, you would add a new related record before then inserting the PDF.
There is a catch here. The easiest way to work with your related table of container fields is to set up a portal, but that "interactive content" optimization option is not supported for container fields inside a portal. There are several possible workarounds for dealing with this.
1) Do not use Insert PDF. Use Insert File. You will not be able to interactively view the contents of your PDF inside the container field, but now you can insert your PDF's directly into the portal rows. If you select the "store a reference" option when inserting, you can open the PDF's by double clicking the container field.
2) Use Insert PDF to insert the file into a container field not in the portal. Use a script with copy the container field's contents to a new portal record and reverse the process each time the user needs to view the PDF from within the container field.
3) Don't use a portal. It's possible to set up a list view layout based on your container field table, but with fields included from your employees table in the header of the layout.