Hello,
you can store this in the PDF file itself with the tool Set PDF viewer preferences: select your PDF, set Page mode to UseThumbs and click Change preferences, then download the new PDF. It then asks the viewer to open with the thumbnails panel.
Online: https://tools.pdf24.org/en/set-pdf-viewer-preferences
In PDF24 Creator you find the same tool in the PDF24 Toolbox as Edit viewer preferences.
Please note: this setting is only a request to the program that opens the file. Each viewer decides itself whether it follows it. Adobe Acrobat Reader, for example, takes it into account. PDF24 Reader currently ignores the page mode stored in the PDF. Supporting it is on our to-do list.
If you meant the thumbnails of PDF files in Windows File Explorer instead, please let us know.