If $f(x) \neq 0$, find a particular solution (usually by guessing a similar form to $f(x)$ or using the $D$-operator method) and add it to the Complementary Function ($y_c$). $$ y = y_c + y_p $$
This command forces PDFtk to read and rebuild the PDF’s page tree, often restoring pages that generic repair tools miss.
If your PDF is password-protected and you legally own a physical copy:
: Many students access the Titas ODE PDF on Scribd , which allows for online viewing or download.