I have a PDF file with 5 pages. I have created an imposed (nup'ped) PDF file with all those pages on one long paper sheet. I used pdfpages LaTeX package for that and the following code:
\includepdf[pages={1-5},nup=1x5]{original.pdf}
The original PDF file had recognized and selectable text on all pages. But in the resulting file (result.pdf) only the first two pages allow text selection. The imposition was done correctly and looks as expected.
It turned out to be the poppler's 50`000 characters limit per page.