In the most simple terms, let's begin with the assumption a user would be satisfied with the preservation of only the PDF copies of a auction catalog, and not the surrounding broad search mechanisms available on many auction house archives. To presume further, we have the "scope creep" mentioned earlier.
The simple answer to your question is that one would first have to "harvest" the PDF of the catalog, and store it on his computer. Each browser has a "download" file option. A user would then perform his own file management of where these PDF files reside on his computer.
Anyone can do this to his personal computer if he desires access to these catalog files and does not want to go online each time he wants to access the catalog(s).
In order for that PDF file to be made available to an external audience, we must now assume that the original host no longer exists, or that prior to the demise of the original website, a formal (legal) agreement had been created between the original owner and the potential new host to replicate the files in the future.
For example, to duplicate the collection of the PDF versions of the Schuyler Rumsey catalogs from No. 19 to No. 105, he would have to first "harvest" each of the catalogs.
To do anything else, i.e., to share the catalog PDF with the "world", it will be assumed that the potential alternate host has an agreement with the Schuyler Rumsey organization to do so. Then the following tasks would be required.
First, the new host would upload each the harvested Schuyler Rumsey catalog PDFs files to his website, then create a page with a link to each catalog. A future user would then simply go to that website, find the page, and find the reference to the link to the catalog he desired. This could be a very simple page, which can be created using any of the commercially available web page editors.
Other approaches are possible.
As a purely hypothetical example, which is not a recommendation, or even a suggestion, I believe it would be theoretically possible for the APRL, if it had the PDFs for these 86 Schuyler Rumsey catalogs available, and had the necessary permission to make them available in the Robert Mason Digital Library, they could treat them in the same manner as they do the various philatelic journals they presently host online, and use that same management software to allow users to access them. The way that the Robert Mason Digital Library handles journals is that they appear to host several copies of each individual page as a JPG file in different resolutions, as well as a full PDF. The user can download in either format. Whether the current software would accommodate only the PDF format is unknown. If not, then this approach would require significant additional work to create all the pages as JPG files.
An alternative would be to create a entry in the David Straight Union Catalog for each individual catalog, then include within the catalog entry a URL link to the individual Schuyler Rumsey auction catalog which would be hosted somewhere on the APRL site. The user would then simply go to the David Straight catalog, find the entry for the SR catalog of interest, and select the link. For that matter, this approach could easily be implemented at the present time, where the URL provided was defined to be the location on the Schuyler Rumsey website where each catalog currently resides.
Edited to remove some redundant statements.