You have to be careful with the type of files that you load onto an SD card if your going to use it in different devices. The Pdf files were they loaded by a computer or another device ? Could you access the pdf files before you loaded these other files ? If the tablet did not recognize the pdf files then they may have written over them not knowing that they were there.
Sorry for the late response.Actual sequence is like this:
1> Got the above said TAB which is budget one and had very less internal memory.
2> Took out 8 GB sd card (class4) from my phone and mounted on WIN7. Cut-Paste all the data to PC.
3> Then from PC copied some PDFs, Office file,Mp3,MKVs into the SD card.Total 3-4 GBs.
4> Tested each file in the TAB and all were accessible and working fine.
5> Then downloaded an app named "Apps2sd" from play store.
6> Using this app Moved some other apps to Sd card. It was successful.
7> Ran all those apps from sd card and found they were working ok.
8> Switch off the tab.
9> After one day when switched on the tab and tryed to access the files from sd card,nothing was there.
10> checked from system setting on the tab: the card was showing 128mb only.
11> tried to mount it on win 7 it was showing it needs to be formatted. tryed but format failed.
12> Tried to low level format using 3 free tools.All failed.
13> Again tried in win7 but no way.