This question is a possible duplicate of A and B.
That feature of “pinning Recent Documents” is currently not implemented in LibreOffice, or in any extension so far as i know.
There is a feature request here, but i saw it has no Assignee yet…
In the mean time you could increase the PickListSize of your Recent Documents list, so that your commonly used files won’t drop off the list after opening many files.
NB. The PickListSize can be set as follows:
1) Open the menu "Tools : Options... : LibreOffice : Advanced";
2) Click on the button "Open Expert Configuration";
3) In the Expert Configuration dialog that pops up, type the word "PickListSize" into the Editbox, and then press the ENTER key or click on the "Search" button;
4) Doubleclick on the node called "History" with the Property "PickListSize";
5) In the dialog that pops up, enter the new value for your PickListSize, e.g. 100.